NO Problem: f(x,f(y,z)) -> f(f(x,y),f(x,z)) f(f(x,y),z) -> f(f(x,z),f(y,z)) f(f(x,y),f(y,z)) -> y Proof: Containment Processor: loop length: 1 terms: f(x,f(y,z)) context: [] substitution: x -> f(x,y) y -> x z -> z Qed