Goal:
  
  G1 : t
  G2 : t
  H : invariants G1
  ext := EdgeSet.diff (E G2) (E G1) : EdgeSet.t
  bs := border_set G1 ext : VSet.t
  s : VSet.t
  y : V.t
  z : V.t
  p : SPath G2 s y z
  ============================
   (forall (s2 : VSet.t) (x0 z0 : V.t) (s1 : VSet.t) (y0 : V.t),
    DisjointAdd x0 s1 s2 ->
    EdgeOf G2 x0 y0 ->
    forall q : SPath G2 s1 y0 z0,
    (forall (x : V.t) (projT1 : Z),
     VSet.In z0 (V G1) ->
     let e := (x, projT1, y0) in
     EdgeSet.In e ext -> VSet.In (first_in G1 G2 q) bs) ->
    VSet.mem x0 (V G1) = false ->
    forall (x : V.t) (projT1 : Z),
    VSet.In z0 (V G1) ->
    let e := (x, projT1, x0) in
    EdgeSet.In e ext -> VSet.In (first_in G1 G2 q) bs)


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