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`isalnum'--alphanumeric character predicate

     #include <ctype.h>
     int isalnum(int C);
`isalnum' is a macro which classifies ASCII integer values by table
lookup.  It is a predicate returning non-zero for alphabetic or numeric
ASCII characters, and `0' for other arguments.  It is defined for all
integer values.
   You can use a compiled subroutine instead of the macro definition by
undefining the macro using ``#undef isalnum''.
`isalnum' returns non-zero if C is a letter (`a'-`z' or `A'-`Z') or a
digit (`0'-`9').
`isalnum' is ANSI C.
   No OS subroutines are required.