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


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