k must be a valid index of string. The string-set! procedure stores char in element
k of string and returns unspecified values.
Passing an immutable string to string-set! should cause an exception with condition type &assertion to be raised.(define (f) (make-string 3 #\*))
Note: Implementors are encouraged to make string-set! run in constant time.
Stores char in every element of the given string and returns unspecified values.