MAYBE Proof: ConCon could not decide confluence of the system. \cite{ALS94}, Theorem 4.1 does not apply. ConCon could not decide whether all critical pairs are joinable or not. cons(x, y') = nil <= sub(0, x) = z', ssp'($2, z') = y': For all conditions c 'tcap(c.lhs)' is unifiable with c.rhs.