Goal:
  
  G1 : t
  ext : EdgeSet.t
  x : VSet.elt
  e : Edge.t
  s : VSet.t
  _b_ : VSet.In x s
  ============================
   (x = e..s \/ x = e..t)


DEBUG:Going to execute:
t_solver
Level 1: Backtrace:

Failed to progress.