fact: Variable lp_XY : length (L++R) = length (L'++R') /\ 1 <= length (L++R).
imports: Relation_Operators Operators_Properties List Lia PeanoNat Undecidability.StackMachines.SMN Undecidability.StackMachines.Util(List_facts SMN_facts) ssreflect ssrbool ssrfun