Goal:
  
  s : VSet.t
  x : VSet.elt
  H : VSet.In x s
  z : VSet.elt
  Hz : VSet.In z s
  e : x = z
  ============================
   (x = z)


DEBUG:Going to execute:
solve
[ trivial
with eq_true
| reflexivity
| symmetry; trivial
| contradiction
| congruence ]