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Control Structures

A Lisp program consists of expressions or forms (see section Kinds of Forms). We control the order of execution of the forms by enclosing them in control structures. Control structures are special forms which control when, whether, or how many times to execute the forms they contain.

The simplest control structure is sequential execution: first form a, then form b, and so on. This is what happens when you write several forms in succession in the body of a function, or at top level in a file of Lisp code--the forms are executed in the order they are written. We call this textual order. For example, if a function body consists of two forms a and b, evaluation of the function evaluates first a and then b, and the function's value is the value of b.

Naturally, Emacs Lisp has many kinds of control structures, including other varieties of sequencing, function calls, conditionals, iteration, and (controlled) jumps. The built-in control structures are special forms since their subforms are not necessarily evaluated. You can use macros to define your own control structure constructs (see section Macros).

Sequencing

Evaluating forms in the order they are written is the most common control structure. Sometimes this happens automatically, such as in a function body. Elsewhere you must use a control structure construct to do this: progn, the simplest control construct of Lisp.

A progn special form looks like this:

(progn a b c ...)

and it says to execute the forms a, b, c and so on, in that order. These forms are called the body of the progn form. The value of the last form in the body becomes the value of the entire progn.

When Lisp was young, progn was the only way to execute two or more forms in succession and use the value of the last of them. But programmers found they often needed to use a progn in the body of a function, where (at that time) only one form was allowed. So the body of a function was made into an "implicit progn": several forms are allowed just as in the body of an actual progn. Many other control structures likewise contain an implicit progn. As a result, progn is not used as often as it used to be. It is needed now most often inside of an unwind-protect, and, or or.

Special Form: progn forms...

This special form evaluates all of the forms, in textual order, returning the result of the final form.

(progn (print "The first form")
       (print "The second form")
       (print "The third form"))
     -| "The first form"
     -| "The second form"
     -| "The third form"
=> "The third form"

Two other control constructs likewise evaluate a series of forms but return a different value:

Special Form: prog1 form1 forms...

This special form evaluates form1 and all of the forms, in textual order, returning the result of form1.

(prog1 (print "The first form")
       (print "The second form")
       (print "The third form"))
     -| "The first form"
     -| "The second form"
     -| "The third form"
=> "The first form"

Here is a way to remove the first element from a list in the variable x, then return the value of that former element:

(prog1 (car x) (setq x (cdr x)))

Special Form: prog2 form1 form2 forms...

This special form evaluates form1, form2, and all of the following forms, in textual order, returning the result of form2.

(prog2 (print "The first form")
       (print "The second form")
       (print "The third form"))
     -| "The first form"
     -| "The second form"
     -| "The third form"
=> "The second form"

Conditionals

Conditional control structures choose among alternatives. Emacs Lisp has two conditional forms: if, which is much the same as in other languages, and cond, which is a generalized case statement.

Special Form: if condition then-form else-forms...

if chooses between the then-form and the else-forms based on the value of condition. If the evaluated condition is non-nil, then-form is evaluated and the result returned. Otherwise, the else-forms are evaluated in textual order, and the value of the last one is returned. (The else part of if is an example of an implicit progn. See section Sequencing.)

If condition has the value nil, and no else-forms are given, if returns nil.

if is a special form because the branch which is not selected is never evaluated--it is ignored. Thus, in the example below, true is not printed because print is never called.

(if nil 
    (print 'true) 
  'very-false)
=> very-false

Special Form: cond clause...

cond chooses among an arbitrary number of alternatives. Each clause in the cond must be a list. The CAR of this list is the condition; the remaining elements, if any, the body-forms. Thus, a clause looks like this:

(condition body-forms...)

cond tries the clauses in textual order, by evaluating the condition of each clause. If the value of condition is non-nil, the body-forms are evaluated, and the value of the last of body-forms becomes the value of the cond. The remaining clauses are ignored.

If the value of condition is nil, the clause "fails", so the cond moves on to the following clause, trying its condition.

If every condition evaluates to nil, so that every clause fails, cond returns nil.

A clause may also look like this:

(condition)

Then, if condition is non-nil when tested, the value of condition becomes the value of the cond form.

The following example has four clauses, which test for the cases where the value of x is a number, string, buffer and symbol, respectively:

(cond ((numberp x) x)
      ((stringp x) x)
      ((bufferp x)
       (setq temporary-hack x) ; multiple body-forms
       (buffer-name x))        ; in one clause
      ((symbolp x) (symbol-value x)))

Often we want the last clause to be executed whenever none of the previous clauses was successful. To do this, we use t as the condition of the last clause, like this: (t body-forms). The form t evaluates to t, which is never nil, so this clause never fails, provided the cond gets to it at all.

For example,

(cond ((eq a 1) 'foo)
      (t "default"))
=> "default"

This expression is a cond which returns foo if the value of a is 1, and returns the string "default" otherwise.

Both cond and if can usually be written in terms of the other. Therefore, the choice between them is a matter of taste and style. For example:

(if a b c)
==
(cond (a b) (t c))

Constructs for Combining Conditions

This section describes three constructs that are often used together with if and cond to express complicated conditions. The constructs and and or can also be used individually as kinds of multiple conditional constructs.

Function: not condition

This function tests for the falsehood of condition. It returns t if condition is nil, and nil otherwise. The function not is identical to null, and we recommend using null if you are testing for an empty list.

Special Form: and conditions...

The and special form tests whether all the conditions are true. It works by evaluating the conditions one by one in the order written.

If any of the conditions evaluates to nil, then the result of the and must be nil regardless of the remaining conditions; so the remaining conditions are ignored and the and returns right away.

If all the conditions turn out non-nil, then the value of the last of them becomes the value of the and form.

Here is an example. The first condition returns the integer 1, which is not nil. Similarly, the second condition returns the integer 2, which is not nil. The third condition is nil, so the remaining condition is never evaluated.

(and (print 1) (print 2) nil (print 3))
     -| 1
     -| 2
=> nil

Here is a more realistic example of using and:

(if (and (consp foo) (eq (car foo) 'x))
    (message "foo is a list starting with x"))

Note that (car foo) is not executed if (consp foo) returns nil, thus avoiding an error.

and can be expressed in terms of either if or cond. For example:

(and arg1 arg2 arg3)
==
(if arg1 (if arg2 arg3))
==
(cond (arg1 (cond (arg2 arg3))))

Special Form: or conditions...

The or special form tests whether at least one of the conditions is true. It works by evaluating all the conditions one by one in the order written.

If any of the conditions evaluates to a non-nil value, then the result of the or must be non-nil; so the remaining conditions are ignored and the or returns right away. The value it returns is the non-nil value of the condition just evaluated.

If all the conditions turn out nil, then the or expression returns nil.

For example, this expression tests whether x is either 0 or nil:

(or (eq x nil) (= x 0))

Like the and construct, or can be written in terms of cond. For example:

(or arg1 arg2 arg3)
==
(cond (arg1)
      (arg2)
      (arg3))

You could almost write or in terms of if, but not quite:

(if arg1 arg1
  (if arg2 arg2 
    arg3))

This is not completely equivalent because it can evaluate arg1 or arg2 twice. By contrast, (or arg1 arg2 arg3) never evaluates any argument more than once.

Iteration

Iteration means executing part of a program repetitively. For example, you might want to repeat some expressions once for each element of a list, or once for each integer from 0 to n. You can do this in Emacs Lisp with the special form while:

Special Form: while condition forms...

while first evaluates condition. If the result is non-nil, it evaluates forms in textual order. Then it reevaluates condition, and if the result is non-nil, it evaluates forms again. This process repeats until condition evaluates to nil.

There is no limit on the number of iterations that may occur. The loop will continue until either condition evaluates to nil or until an error or throw jumps out of it (see section Nonlocal Exits).

The value of a while form is always nil.

(setq num 0)
     => 0
(while (< num 4)
  (princ (format "Iteration %d." num))
  (setq num (1+ num)))
     -| Iteration 0.
     -| Iteration 1.
     -| Iteration 2.
     -| Iteration 3.
     => nil

If you would like to execute something on each iteration before the end-test, put it together with the end-test in a progn as the first argument of while, as shown here:

(while (progn
         (forward-line 1)
         (not (looking-at "^$"))))

This moves forward one line and continues moving by lines until an empty line is reached.

Nonlocal Exits

A nonlocal exit is a transfer of control from one point in a program to another remote point. Nonlocal exits can occur in Emacs Lisp as a result of errors; you can also use them under explicit control. Nonlocal exits unbind all variable bindings made by the constructs being exited.

Explicit Nonlocal Exits: catch and throw

Most control constructs affect only the flow of control within the construct itself. The function throw is the exception to this rule for of normal program execution: it performs a nonlocal exit on request. (There are other exceptions, but they are for error handling only.) throw is used inside a catch, and jumps back to that catch. For example:

(catch 'foo
  (progn
    ...
      (throw 'foo t)
    ...))

The throw transfers control straight back to the corresponding catch, which returns immediately. The code following the throw is not executed. The second argument of throw is used as the return value of the catch.

The throw and the catch are matched through the first argument: throw searches for a catch whose first argument is eq to the one specified. Thus, in the above example, the throw specifies foo, and the catch specifies the same symbol, so that catch is applicable. If there is more than one applicable catch, the innermost one takes precedence.

All Lisp constructs between the catch and the throw, including function calls, are exited automatically along with the catch. When binding constructs such as let or function calls are exited in this way, the bindings are unbound, just as they are when these constructs are exited normally (see section Local Variables). Likewise, the buffer and position saved by save-excursion (see section Excursions) are restored, and so is the narrowing status saved by save-restriction and the window selection saved by save-window-excursion (see section Window Configurations). Any cleanups established with the unwind-protect special form are executed if the unwind-protect is exited with a throw.

The throw need not appear lexically within the catch that it jumps to. It can equally well be called from another function called within the catch. As long as the throw takes place chronologically after entry to the catch, and chronologically before exit from it, it has access to that catch. This is why throw can be used in commands such as exit-recursive-edit which throw back to the editor command loop (see section Recursive Editing).

Common Lisp note: most other versions of Lisp, including Common Lisp, have several ways of transferring control nonsequentially: return, return-from, and go, for example. Emacs Lisp has only throw.

Special Form: catch tag body...

catch establishes a return point for the throw function. The return point is distinguished from other such return points by tag, which may be any Lisp object. The argument tag is evaluated normally before the return point is established.

With the return point in effect, the forms of the body are evaluated in textual order. If the forms execute normally, without error or nonlocal exit, the value of the last body form is returned from the catch.

If a throw is done within body specifying the same value tag, the catch exits immediately; the value it returns is whatever was specified as the second argument of throw.

Function: throw tag value

The purpose of throw is to return from a return point previously established with catch. The argument tag is used to choose among the various existing return points; it must be eq to the value specified in the catch. If multiple return points match tag, the innermost one is used.

The argument value is used as the value to return from that catch.

If no return point is in effect with tag tag, then a no-catch error is signaled with data (tag value).

Examples of catch and throw

One way to use catch and throw is to exit from a doubly nested loop. (In most languages, this would be done with a "go to".) Here we compute (foo i j) for i and j varying from 0 to 9:

(defun search-foo ()
  (catch 'loop
    (let ((i 0))
      (while (< i 10)
        (let ((j 0))
          (while (< j 10)
            (if (foo i j)
                (throw 'loop (list i j)))
            (setq j (1+ j))))
        (setq i (1+ i))))))

If foo ever returns non-nil, we stop immediately and return a list of i and j. If foo always returns nil, the catch returns normally, and the value is nil, since that is the result of the while.

Here are two tricky examples, slightly different, showing two return points at once. First, two return points with the same tag, hack:

(defun catch2 (tag)
  (catch tag
    (throw 'hack 'yes)))
=> catch2

(catch 'hack 
  (print (catch2 'hack))
  'no)
-| yes
=> no

Since both return points have tags that match the throw, it goes to the inner one, the one established in catch2. Therefore, catch2 returns normally with value yes, and this value is printed. Finally the second body form in the outer catch, which is 'no, is evaluated and returned from the outer catch.

Now let's change the argument given to catch2:

(defun catch2 (tag)
  (catch tag
    (throw 'hack 'yes)))
=> catch2

(catch 'hack
  (print (catch2 'quux))
  'no)
=> yes

We still have two return points, but this time only the outer one has the tag hack; the inner one has the tag quux instead. Therefore, the throw returns the value yes from the outer return point. The function print is never called, and the body-form 'no is never evaluated.

Errors

When Emacs Lisp attempts to evaluate a form that, for some reason, cannot be evaluated, it signals an error.

When an error is signaled, Emacs's default reaction is to print an error message and terminate execution of the current command. This is the right thing to do in most cases, such as if you type C-f at the end of the buffer.

In complicated programs, simple termination may not be what you want. For example, the program may have made temporary changes in data structures, or created temporary buffers which should be deleted before the program is finished. In such cases, you would use unwind-protect to establish cleanup expressions to be evaluated in case of error. Occasionally, you may wish the program to continue execution despite an error in a subroutine. In these cases, you would use condition-case to establish error handlers to recover control in case of error.

Resist the temptation to use error handling to transfer control from one part of the program to another; use catch and throw. See section Explicit Nonlocal Exits: catch and throw.

How to Signal an Error

Most errors are signaled "automatically" within Lisp primitives which you call for other purposes, such as if you try to take the CAR of an integer or move forward a character at the end of the buffer; you can also signal errors explicitly with the functions error and signal.

Quitting, which happens when the user types C-g, is not considered an error, but it handled almost like an error. See section Quitting.

Function: error format-string &rest args

This function signals an error with an error message constructed by applying format (see section Conversion of Characters and Strings) to format-string and args.

Typical uses of error is shown in the following examples:

(error "You have committed an error.  
        Try something else.")
     error--> You have committed an error.  
        Try something else.

(error "You have committed %d errors." 10)
     error--> You have committed 10 errors.  

error works by calling signal with two arguments: the error symbol error, and a list containing the string returned by format.

If you want to use a user-supplied string as an error message verbatim, don't just write (error string). If string contains `%', it will be interpreted as a format specifier, with undesirable results. Instead, use (error "%s" string).

Function: signal error-symbol data

This function signals an error named by error-symbol. The argument data is a list of additional Lisp objects relevant to the circumstances of the error.

The argument error-symbol must be an error symbol---a symbol bearing a property error-conditions whose value is a list of condition names. This is how different sorts of errors are classified.

The number and significance of the objects in data depends on error-symbol. For example, with a wrong-type-arg error, there are two objects in the list: a predicate which describes the type that was expected, and the object which failed to fit that type. See section Error Symbols and Condition Names, for a description of error symbols.

Both error-symbol and data are available to any error handlers which handle the error: a list (error-symbol . data) is constructed to become the value of the local variable bound in the condition-case form (see section Writing Code to Handle Errors). If the error is not handled, both of them are used in printing the error message.

The function signal never returns (though in older Emacs versions it could sometimes return).

(signal 'wrong-number-of-arguments '(x y))
     error--> Wrong number of arguments: x, y

(signal 'no-such-error '("My unknown error condition."))
     error--> peculiar error: "My unknown error condition."

Common Lisp note: Emacs Lisp has nothing like the Common Lisp concept of continuable errors.

How Emacs Processes Errors

When an error is signaled, Emacs searches for an active handler for the error. A handler is a specially marked place in the Lisp code of the current function or any of the functions by which it was called. If an applicable handler exists, its code is executed, and control resumes following the handler. The handler executes in the environment of the condition-case which established it; all functions called within that condition-case have already been exited, and the handler cannot return to them.

If no applicable handler is in effect in your program, the current command is terminated and control returns to the editor command loop, because the command loop has an implicit handler for all kinds of errors. The command loop's handler uses the error symbol and associated data to print an error message.

When an error is not handled explicitly, it may cause the Lisp debugger to be called. The debugger is enabled if the variable debug-on-error (see section Entering the Debugger on an Error) is non-nil. Unlike error handlers, the debugger runs in the environment of the error, so that you can examine values of variables precisely as they were at the time of the error.

Writing Code to Handle Errors

The usual effect of signaling an error is to terminate the command that is running and return immediately to the Emacs editor command loop. You can arrange to trap errors occurring in a part of your program by establishing an error handler with the special form condition-case. A simple example looks like this:

(condition-case nil
    (delete-file filename)
  (error nil))

This deletes the file named filename, catching any error and returning nil if an error occurs.

The second argument of condition-case is called the protected form. (In the example above, the protected form is a call to delete-file.) The error handlers go into effect when this form begins execution and are deactivated when this form returns. They remain in effect for all the intervening time. In particular, they are in effect during the execution of subroutines called by this form, and their subroutines, and so on. This is a good thing, since, strictly speaking, errors can be signaled only by Lisp primitives (including signal and error) called by the protected form, not by the protected form itself.

The arguments after the protected form are handlers. Each handler lists one or more condition names (which are symbols) to specify which errors it will handle. The error symbol specified when an error is signaled also defines a list of condition names. A handler applies to an error if they have any condition names in common. In the example above, there is one handler, and it specifies one condition name, error, which covers all errors.

The search for an applicable handler checks all the established handlers starting with the most recently established one. Thus, if two nested condition-case forms try to handle the same error, the inner of the two will actually handle it.

When an error is handled, control returns to the handler. Before this happens, Emacs unbinds all variable bindings made by binding constructs that are being exited and executes the cleanups of all unwind-protect forms that are exited. Once control arrives at the handler, the body of the handler is executed.

After execution of the handler body, execution continues by returning from the condition-case form. Because the protected form is exited completely before execution of the handler, the handler cannot resume execution at the point of the error, nor can it examine variable bindings that were made within the protected form. All it can do is clean up and proceed.

condition-case is often used to trap errors that are predictable, such as failure to open a file in a call to insert-file-contents. It is also used to trap errors that are totally unpredictable, such as when the program evaluates an expression read from the user.

Error signaling and handling have some resemblance to throw and catch, but they are entirely separate facilities. An error cannot be caught by a catch, and a throw cannot be handled by an error handler (though using throw when there is no suitable catch signals an error which can be handled).

Special Form: condition-case var protected-form handlers...

This special form establishes the error handlers handlers around the execution of protected-form. If protected-form executes without error, the value it returns becomes the value of the condition-case form; in this case, the condition-case has no effect. The condition-case form makes a difference when an error occurs during protected-form.

Each of the handlers is a list of the form (conditions body...). conditions is an error condition name to be handled, or a list of condition names; body is one or more Lisp expressions to be executed when this handler handles an error. Here are examples of handlers:

(error nil)

(arith-error (message "Division by zero"))

((arith-error file-error)
 (message
  "Either division by zero or failure to open a file"))

Each error that occurs has an error symbol which describes what kind of error it is. The error-conditions property of this symbol is a list of condition names (see section Error Symbols and Condition Names). Emacs searches all the active condition-case forms for a handler which specifies one or more of these names; the innermost matching condition-case handles the error. The handlers in this condition-case are tested in the order in which they appear.

The body of the handler is then executed, and the condition-case returns normally, using the value of the last form in the body as the overall value.

The argument var is a variable. condition-case does not bind this variable when executing the protected-form, only when it handles an error. At that time, var is bound locally to a list of the form (error-symbol . data), giving the particulars of the error. The handler can refer to this list to decide what to do. For example, if the error is for failure opening a file, the file name is the second element of data---the third element of var.

If var is nil, that means no variable is bound. Then the error symbol and associated data are not made available to the handler.

Here is an example of using condition-case to handle the error that results from dividing by zero. The handler prints out a warning message and returns a very large number.

(defun safe-divide (dividend divisor)
  (condition-case err                
      ;; Protected form.
      (/ dividend divisor)              
    ;; The handler.
    (arith-error                        ; Condition.
     (princ (format "Arithmetic error: %s" err))
     1000000)))
=> safe-divide

(safe-divide 5 0)
     -| Arithmetic error: (arith-error)
=> 1000000

The handler specifies condition name arith-error so that it will handle only division-by-zero errors. Other kinds of errors will not be handled, at least not by this condition-case. Thus,

(safe-divide nil 3)
     error--> Wrong type argument: integer-or-marker-p, nil

Here is a condition-case that catches all kinds of errors, including those signaled with error:

(setq baz 34)
     => 34

(condition-case err
    (if (eq baz 35)
        t
      ;; This is a call to the function error.
      (error "Rats!  The variable %s was %s, not 35." 'baz baz))
  ;; This is the handler; it is not a form.
  (error (princ (format "The error was: %s" err)) 
         2))
-| The error was: (error "Rats!  The variable baz was 34, not 35.")
=> 2

Error Symbols and Condition Names

When you signal an error, you specify an error symbol to specify the kind of error you have in mind. Each error has one and only one error symbol to categorize it. This is the finest classification of errors defined by the Lisp language.

These narrow classifications are grouped into a hierarchy of wider classes called error conditions, identified by condition names. The narrowest such classes belong to the error symbols themselves: each error symbol is also a condition name. There are also condition names for more extensive classes, up to the condition name error which takes in all kinds of errors. Thus, each error has one or more condition names: error, the error symbol if that is distinct from error, and perhaps some intermediate classifications.

In order for a symbol to be usable as an error symbol, it must have an error-conditions property which gives a list of condition names. This list defines the conditions which this kind of error belongs to. (The error symbol itself, and the symbol error, should always be members of this list.) Thus, the hierarchy of condition names is defined by the error-conditions properties of the error symbols.

In addition to the error-conditions list, the error symbol should have an error-message property whose value is a string to be printed when that error is signaled but not handled. If the error-message property exists, but is not a string, the error message `peculiar error' is used.

Here is how we define a new error symbol, new-error:

(put 'new-error
     'error-conditions
     '(error my-own-errors new-error))       
     => (error my-own-errors new-error)
(put 'new-error 'error-message "A new error")
     => "A new error"

This error has three condition names: new-error, the narrowest classification; my-own-errors, which we imagine is a wider classification; and error, which is the widest of all. Naturally, Emacs will never signal a new-error on its own; only an explicit call to signal (see section Errors) in your code can do this:

(signal 'new-error '(x y))
     error--> A new error: x, y

This error can be handled through any of the three condition names. This example handles new-error and any other errors in the class my-own-errors:

(condition-case foo
    (bar nil t)
  (my-own-errors nil))

The significant way that errors are classified is by their condition names--the names used to match errors with handlers. An error symbol serves only as a convenient way to specify the intended error message and list of condition names. If signal were given a list of condition names rather than one error symbol, that would be cumbersome.

By contrast, using only error symbols without condition names would seriously decrease the power of condition-case. Condition names make it possible to categorize errors at various levels of generality when you write an error handler. Using error symbols alone would eliminate all but the narrowest level of classification.

See section Standard Errors, for a list of all the standard error symbols and their conditions.

Cleaning Up from Nonlocal Exits

The unwind-protect construct is essential whenever you temporarily put a data structure in an inconsistent state; it permits you to ensure the data are consistent in the event of an error or throw.

Special Form: unwind-protect body cleanup-forms...

unwind-protect executes the body with a guarantee that the cleanup-forms will be evaluated if control leaves body, no matter how that happens. The body may complete normally, or execute a throw out of the unwind-protect, or cause an error; in all cases, the cleanup-forms will be evaluated.

Only the body is actually protected by the unwind-protect. If any of the cleanup-forms themselves exit nonlocally (e.g., via a throw or an error), it is not guaranteed that the rest of them will be executed. If the failure of one of the cleanup-forms has the potential to cause trouble, then it should be protected by another unwind-protect around that form.

The number of currently active unwind-protect forms counts, together with the number of local variable bindings, against the limit max-specpdl-size (see section Local Variables).

For example, here we make an invisible buffer for temporary use, and make sure to kill it before finishing:

(save-excursion
  (let ((buffer (get-buffer-create " *temp*")))
    (set-buffer buffer)
    (unwind-protect
        body
      (kill-buffer buffer))))

You might think that we could just as well write (kill-buffer (current-buffer)) and dispense with the variable buffer. However, the way shown above is safer, if body happens to get an error after switching to a different buffer! (Alternatively, you could write another save-excursion around the body, to ensure that the temporary buffer becomes current in time to kill it.)

Here is an actual example taken from the file `ftp.el'. It creates a process (see section Processes) to try to establish a connection to a remote machine. As the function ftp-login is highly susceptible to numerous problems which the writer of the function cannot anticipate, it is protected with a form that guarantees deletion of the process in the event of failure. Otherwise, Emacs might fill up with useless subprocesses.

(let ((win nil))
  (unwind-protect
      (progn
        (setq process (ftp-setup-buffer host file))
        (if (setq win (ftp-login process host user password))
            (message "Logged in")
          (error "Ftp login failed")))
    (or win (and process (delete-process process)))))

This example actually has a small bug: if the user types C-g to quit, and the quit happens immediately after the function ftp-setup-buffer returns but before the variable process is set, the process will not be killed. There is no easy way to fix this bug, but at least it is very unlikely.

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