Goal:
  
  s : VSet.t
  x : VSet.elt
  H : VSet.In x s
  z : VSet.elt
  Hz : VSet.In z s
  n : x <> z
  ============================
   (VSet.In z s)


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