Goal:
    G1 : t  G2 : t  s : VSet.t  x : V.t  y : V.t  p : SPath G2 s x y
  ============================
   (VSet.In x (V G1) -> first_in p = x)


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