Goal:
    G : t  HI : invariants G  HG : acyclic_no_loop G  n : Z  x : V.t  y : V.t
  ============================
   ((if VSet.mem y (V G)
     then (Some n <= lsp G x y)%nbar
     else n <= 0 /\ (x = y \/ (Some n <= lsp G x (s G))%nbar)) <->
    leqb_vertices n x y)


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrrewrite@0> $1 $2
DEBUG LTAC Evaluated term: leq_vertices_caract