Goal:
    x : V.t  y : V.t  p : VSet.t  a : V.t  H0 : x = a
  ============================
   (y = a \/ x = a \/ VSet.In a p)


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