Success: occurs(a,a). occurs(X,f(a,b,c,X)). occurs(+,f(+,-)). occurs(a,[b,c,a,g,a]). occurs([ ],[a,b]). occurs(1,[A|1]). occurs(1.0,[1.0|B]). Fail: occurs(a,b). occurs(X,f(Y,Z)). occurs(X,Y). occurs(1,"2314"). occurs([], [a,b|c]). Error: occurs("str",f("str1","str2","str")). (Error 5) occurs([a],[a,b]). (Error 5) occurs(f(a,b),f(a,b)). (Error 5)