StringReplaceSubstringFn is an evaluatable function that takes three strings as its arguments. (StringReplaceSubstringFn STRING SUBSTRING NEWSTRING) tests to make sure that the n tokens in SUBSTRING exactly match a substring of STRING. If this condition is met, the function will then replace all occurrences of SUBSTRING in STRING with NEWSTRING, returning the resulting string. Otherwise the function will simply return STRING. For example, (StringReplaceSubstringFn 'I like to bicycle' 'like' 'hate') returns 'I hate to bicycle'. However, (StringReplaceSubstringFn 'I like to bicycle' 'likes' 'hate') would return 'I like to bicycle'.