concatenate two strings
Major Section:  PROGRAMMING

NOTE: It is probably more efficient to use the Common Lisp function concatenate in place of string-append. That is,

(string-append str1 str2)
is equal to
(concatenate 'string str1 str2).

At any rate, string-append takes two arguments, which are both strings (if the guard is to be met), and returns a string obtained concatenating together the characters in the first string followed by those in the second. See concatenate.