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 : a = x
  H4 : VSet.In a s0 -> VSet.In a s'
  H3 : VSet.In a s'
  ============================
   (VSet.In a s0)


DEBUG:Going to execute:
contradiction
Level 1: Backtrace:

 The relation VSet.In is not a declared symmetric relation. Maybe you need to require the Coq.Classes.RelationClasses library
Level 0: Backtrace:

 The relation VSet.In is not a declared symmetric relation. Maybe you need to require the Coq.Classes.RelationClasses library
Level 0: Backtrace:

 The relation VSet.In is not a declared symmetric relation. Maybe you need to require the Coq.Classes.RelationClasses library