Goal:
  
  G1 : t
  G2 : t
  H : invariants G1
  ext := EdgeSet.diff (E G2) (E G1) : EdgeSet.t
  bs := border_set G1 ext : VSet.t
  H0 : invariants G2
  H1 : acyclic_no_loop G1
  H2 : acyclic_no_loop G2
  h : forall x y : VSet.elt,
      VSet.In x bs -> VSet.In y bs -> (lsp G2 x y <= lsp G1 x y)%nbar
  x : VSet.elt
  y : VSet.elt
  hx : VSet.In x (V G1)
  hy : VSet.In y (V G1)
  _a_ : Z
  p : SPath G2 (V G2) x y
  hp : sweight p = _a_
  ============================
   ((Some (sweight p) <=
     lsp G2 x (first_in G1 G2 p) + lsp G1 (first_in G1 G2 p) y)%nbar ->
    (Some _a_ <= lsp G1 x y)%nbar)


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