Goal:
  
  A : Type
  P : A → Prop
  a : A
  l : list A
  H : ∀ x : A, In x (a :: l) → P x
  IHl : (∀ x : A, In x l → P x) → Forall P l
  ============================
   (a = a ∨ In a l)


DEBUG:Going to execute:
solve [ intuition eauto  3 with core terms ]
Level 0: Backtrace:

Tactic failure: Cannot solve this goal.
Level 5: Backtrace:

Tactic failure: Cannot solve this goal.
Level 4: Backtrace:

Tactic failure: Cannot solve this goal.
Level 4: Backtrace:

Tactic failure: Cannot solve this goal.
Level 3: Backtrace:

Tactic failure: Cannot solve this goal.
Level 2: Backtrace:

Tactic failure: Cannot solve this goal.
Level 1: Backtrace:

Tactic failure: Cannot solve this goal.
Level 0: Backtrace:

Tactic failure: Cannot solve this goal.