Goal:
  
  G : t
  x : V.t
  i : V.t
  H : x = i \/ False
  source_vertex0 : VSet.In (s G) (V G)
  ============================
   (False \/ i = x)


DEBUG:Going to execute:
repeat
            match goal with
            | H:context [ ?P -> False ] |- _ => fold (~ P) in H
            | |- context [ ?P -> False ] => fold (~ P)
            end