SUBSTRINGP thing1 thing2
SUBSTRING? thing1 thing2

If thing2 is a word, SUBSTRINGP outputs TRUE if thing1 is EQUALP to a substring of thing2, FALSE otherwise.

If thing1 or thing2 is a list or an array, SUBSTRINGP outputs FALSE.

SHOW SUBSTRINGP "bc "abc
true

SHOW SUBSTRINGP "bd "abc
false