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`isxdigit'--hexadecimal digit predicate

     #include <ctype.h>
     int isxdigit(int C);
`isxdigit' is a macro which classifies ASCII integer values by table
lookup.  It is a predicate returning non-zero for hexadecimal digits,
and `0' for other characters.  It is defined only when `isascii'(C) is
true or C is EOF.
   You can use a compiled subroutine instead of the macro definition by
undefining the macro using ``#undef isxdigit''.
`isxdigit' returns non-zero if C is a hexadecimal digit (`0'-`9',
`a'-`f', or `A'-`F').
`isxdigit' is ANSI C.
   No supporting OS subroutines are required.