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─────Run-Time Library───────────────────────────────────────────────────────
 
     The routines in the is family test a given integer value. They
     return a nonzero value if the integer satisfies the test
     condition and 0 if it does not. An ASCII-character-set
     environment is assumed. These routines are implemented both as
     functions and as macros.
 
     The __isascii routine produces meaningful results for all integer
     values. However, the remaining routines produce a defined result
     only for integer values corresponding to the ASCII character set
     (that is, only where __isascii holds true) or for the non-ASCII
     value EOF (defined in STDIO.H).
 
     Return Value
 
     All of these functions return a nonzero value if the tested
     character is in the right category and 0 if not.
                                    -♦-