#t if obj is the empty string, otherwise return
Check to see if the given criteria is true on every/any character in str, proceeding from left (index start) to right (index past).
If the selected substring is empty, the return value is
If char/char-set/pred is a character, it is tested for equality with the elements of S.
If char/char-set/pred is a character set, the elements of S are tested for membership in the set.
If char/char-set/pred is a predicate procedure, it is applied to the elements of S. The predicate is “witness–generating”:
string-anyreturns true, the returned true value is the one produced by the application of the predicate.
string-everyreturns true, the returned true value is the one produced by the application of the predicate to the last code point in the substring.
If the predicate is applied to the final element of the selected substring, that final application is a tail call.
The names of these procedures do not end with a question mark; this is
to indicate that, in the predicate case, they do not return a simple
#f), but a general value.