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These functions operate upon single characters of type char.  The functions
test characters in various ways and convert them between upper and
lowercase.  The following functions are defined:

isalnum
    test for letter or digit

isalpha
    test for letter

isascii
    test for ASCII character

iscntrl
    test for control character

isdigit
    test for digit

isgraph
    test for printable character, except space

islower
    test for letter in lowercase

isprint
    test for printable character, including space

ispunct
    test for punctuation characters

isspace
    test for "white space" characters

isupper
    test for letter in uppercase

isxdigit
    test for hexadecimal digit

tolower
    convert character to lowercase

toupper
    convert character to uppercase