string-trim character-bag string ⇒  trimmed-string
string-left-trim character-bag string ⇒  trimmed-string
string-right-trim character-bag string ⇒  trimmed-string
| character-bag | a sequence containing characters. | 
| string | a string designator. | 
| trimmed-string | a string. | 
string-trim returns a substring of string, with all characters in character-bag stripped off the beginning and end. string-left-trim is similar but strips characters off only the beginning; string-right-trim strips off only the end.
If no characters need to be trimmed from the string, then either string itself or a copy of it may be returned, at the discretion of the implementation.
All of these functions observe the fill pointer.
 (string-trim "abc" "abcaakaaakabcaaa") ⇒  "kaaak"
 (string-trim '(#\Space #\Tab #\Newline) " garbanzo beans
        ") ⇒  "garbanzo beans"
 (string-trim " (*)" " ( *three (silly) words* ) ")
⇒  "three (silly) words"
 (string-left-trim "abc" "labcabcabc") ⇒  "labcabcabc"
 (string-left-trim " (*)" " ( *three (silly) words* ) ")
⇒  "three (silly) words* ) "
 (string-right-trim " (*)" " ( *three (silly) words* ) ") 
⇒  " ( *three (silly) words"
The implementation.