Used to find all possible solutions for the concatenation of String1 and String2 to make String3.
Note that if String1 and String2 are instantiated, it is more efficient to use the predicate concat_strings/3.
Success: append_strings("a",B,"abc"). (gives B = "bc"). append_strings(A,B,"a"). (gives A="" B="a"; A="a" B=""). Fail: append_strings("a","b","abc"). Error: append_strings(A,"bc",C). (Error 4). append_strings(5,B,C). (Error 5). append_strings(A,'me',"meme"). (Error 5).