string-left-trim

(string-left-trim character-bag string)

Returns a new string with leading characters that appear in character-bag removed from the front only; the right 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 first character not in the bag. string is a string designator, so (string-left-trim "F" '|FOO|) returns "OO".