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 (x : V.t) (projT1 : Z),
    VSet.In z (V G1) ->
    let e := (x, projT1, y) in
    EdgeSet.In e ext -> VSet.In (first_in G1 G2 p) bs)


DEBUG:Going to execute:
constr:(fun_elim)
Level 5: c
 has value 
(first_in G1 G2 p)