DIGIT-CHAR-P
============

Syntax: (digit-char-p <character>)

This function checks, whether the argument is a digit
character. It returns the digit character, when the
test was successful, otherwise NIL.

uppercase-character:                   A...Z
lowercase-character:                   a...z
digit-character:                       0...9
alphanumeric-character:                A...Z, a...z
                                       and 0...9

example:       >(digit-char-p #\5)
               5