Goal:
  
  s0 : VSet.t
  s1 : VSet.t
  s2 : VSet.t
  x : V.t
  y : V.t
  H1 : DisjointAdd x s0 s1
  H2 : DisjointAdd y s1 s2
  z : VSet.elt
  Hz : VSet.E.eq z y
  ============================
   (y = z)


DEBUG:Going to execute:
reflexivity
Level 0: Backtrace:

Proof is not complete.
Level 0: Backtrace:

Proof is not complete.