If Code is instantiated to an integer, Char is unified with the corresponding one-character atom.
Success: char_code(b,98). char_code("b",98). char_code(C,99). (gives C=c). char_code(a,I). (gives I=97). Fail: char_code(a,98). Error: char_code(C,I). (Error 4). char_code(ab,I). (Error 5). char_code(7,I). (Error 5). char_code(C,-1). (Error 6).