char *strtok( char *str1, const char *str2 )

	- prototype in string.h

	- parses str1 for tokens and separators, where separators are
	  defined in str2
	- first call returns a pointer to the first character of the first
	  token in str1 and writes a null character into str1 immediately
	  following the returned token;   subsequent calls with NULL for
	  the first argument will work through the string str1 in this way
	  until no tokens remain
	- when tokens are exhausted, returns NULL