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
  ============================
   (forall (s s' : VSet.t) (x y z : V.t) (d : DisjointAdd x s s')
      (e : EdgeOf G2 x y) (s0 : SPath G2 s y z),
    (VSet.In z (V G1) ->
     (Some (sweight s0) <=
      lsp G2 y (first_in G1 G2 s0) + lsp G1 (first_in G1 G2 s0) z)%nbar) ->
    VSet.In z (V G1) ->
    (Some (sweight (spath_step G2 s s' x y z d e s0)) <=
     lsp G2 x (first_in G1 G2 (spath_step G2 s s' x y z d e s0)) +
     lsp G1 (first_in G1 G2 (spath_step G2 s s' x y z d e s0)) z)%nbar)


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrmove@1> $1 $2