Goal:
    x : V.t  s : VSet.t  y : V.t  H0 : x = y
  ============================
   ((x = y \/ VSet.In y s -> x = y \/ VSet.In y s /\ x <> y) /\
    (x = y \/ VSet.In y s /\ x <> y -> x = y \/ VSet.In y s))


DEBUG:Going to execute:
fun id0 id1 X2 => assert X2; [ exact (id0 id1) | clear id0 ]