Goal:
  
  map_In : ∀ (A B : Type) (l : list A), (∀ x : A, In x l → B) → list B
  A : Type
  B : Type
  x0 : A
  xs : list A
  f : ∀ x : A, In x (x0 :: xs) → B
  x : A
  H : In x xs
  ============================
   (x0 = x ∨ In x xs)


DEBUG:Going to execute:
try (solve
            [ red; intros; Tactics.destruct_conjs; Tactics.autoinjections;
               Tactics.discriminates ])
Level 2: Backtrace:

Unable to find an instance for the variables A, B, l.
Level 2: Backtrace:

Unable to find an instance for the variables A, B, l.
Level 1: Backtrace:

Unable to find an instance for the variables A, B, l.
Level 0: Backtrace:

Unable to find an instance for the variables A, B, l.