fact: Definition MPCP' '(c, C) := exists I, I ⊆ nats (length C) /\ hd c ++ @concat symbol (select I (heads C)) = tl c ++ concat (select I (tails C)).
imports: RelationClasses Morphisms List Lia Init.Nat Setoid Undecidability.HOU(std.std calculus.calculus unification.unification) Undecidability.HOU(third_order.pcp third_order.encoding)