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


DEBUG:Going to execute:
k ltac:(uconstr:((elimc a b c d)))
Level 7: c
 has value 
(first_in p)