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:
match goal with
           | |- ?X1 -> ?X2 => intro
           | |- ~ _ => unfold not at 1; intro
           end