fact: Variable unique_step_l : forall l r y, step M (l, r, X) y -> l = [A] ++ skipn 1 l.
imports: Relation_Operators Operators_Properties List Lia PeanoNat Undecidability.StackMachines.SMN Undecidability.StackMachines.Util(List_facts SMN_facts) ssreflect ssrbool ssrfun