Goal:
  
  A : Type
  P : A → Type
  x : A
  x0 : A
  l : list A
  H : x0 = x ∨ In x l
  p : P x0
  X : All P l
  IHX : In x l → ∥ P x ∥
  ============================
   (P x)


DEBUG:Going to execute:
<coq-core.plugins.ltac::contradiction@0> $1
Level 0: Backtrace:

No primitive equality found.