(VAR X X1 X2 X3) (RULES active(f(a,b,X)) -> mark(f(X,X,X)) active(c) -> mark(a) active(c) -> mark(b) active(f(X1,X2,X3)) -> f(X1,X2,active(X3)) f(X1,X2,mark(X3)) -> mark(f(X1,X2,X3)) proper(f(X1,X2,X3)) -> f(proper(X1),proper(X2),proper(X3)) proper(a) -> ok(a) proper(b) -> ok(b) proper(c) -> ok(c) f(ok(X1),ok(X2),ok(X3)) -> ok(f(X1,X2,X3)) top(mark(X)) -> top(proper(X)) top(ok(X)) -> top(active(X)) )