Goal:
  
  A : Type
  P : list A → Type
  X : P []
  X0 : ∀ (l : list A) (x : A), P (l ++ [x])
  l : list A
  ============================
   (∀ (x : A) (l : list A), P l → P (l ++ [x]))


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