fact: Definition echelon c := slist_E c (Nat \times Nat) /\ forall i, i <c (slength c) -> let a:= P (Vg c i) in let b:= Q (Vg c i) in (b = \0c -> ech_good a i) /\ (b <> \0c -> a <> \0c -> ech_good a i /\ ech_good b i).
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat) mathcomp(seq)