string-right-trim

(string-right-trim character-bag string)

Returns a new string with trailing characters that appear in character-bag removed from the end only; the left end is left intact. character-bag is any sequence of characters -- a string, a list, or a vector -- whose members form the set to strip, and trimming stops at the last character not in the bag. string is a string designator, so (string-right-trim "O" '|FOO|) returns "F".