Goal:
  
  s0 : VSet.t
  s' : VSet.t
  x : V.t
  d : DisjointAdd x s0 s'
  a : VSet.elt
  H0 : VSet.In a s'
  H1 : VSet.E.eq a x -> False
  H2 : VSet.In a s' -> x = a \/ VSet.In a s0
  H3 : x = a \/ VSet.In a s0 -> VSet.In a s'
  ============================
   (VSet.In a s0)


DEBUG:Going to execute:
assert X2
Level 1: evaluation returns
assert X2; [ exact (id0 id1) | clear id0 ]
where
id1 := H0
of type
intropattern
id0 := H2
of type
intropattern
flags := 
of type
tauto_flags
X2 := x = a \/ VSet.In a s0
of type
constr of type tacvalue