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: Backtrace:


Could not find an instance for "FunctionalElimination 
                                  (@first_in s) ?fun_elim_ty 
                                  ?n"
in environment:
G1, G2 : t
s : VSet.t
x, y : V.t
p : SPath G2 s x y