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:
match goal with
           | H:∃ y, _ y |- _ => tac H
           | H:{x : _ | ?P x} |- _ => tac H
           | H:∑ y, ?P y |- _ => tacT H
           | H:exists2 x : _, _ x & _ x |- _ => tac2 H
           | H:{x : _ | ?P x & _ x} |- _ => tac2 H
           | H:{x : _ & ?P x & _ x} |- _ => tacT2 H
           end