Goal:
  
  A : Type
  B : Type
  f : ∀ x : A, In x [] → B
  A0 : Type
  B0 : Type
  f0 : A0 → B0
  l : list A0
  g : ∀ x : A0, In x l → B0
  ============================
   (g = (λ (x : A0) (_ : In x l), f0 x)
    → ∀ eqargs : {|
                   pr1 := A;
                   pr2 := {| pr1 := B; pr2 := {| pr1 := []; pr2 := f |} |}
                 |} =
                 {|
                   pr1 := A0;
                   pr2 := {| pr1 := B0; pr2 := {| pr1 := l; pr2 := g |} |}
                 |},
        eq_rect
          {|
            pr1 := A; pr2 := {| pr1 := B; pr2 := {| pr1 := []; pr2 := f |} |}
          |}
          (λ projs : sigma
                       (λ A : Type,
                          sigma
                            (λ B : Type,
                               sigma (λ l : list A, ∀ x : A, In x l → B))),
             list (pr1 (pr2 projs))) []
          {|
            pr1 := A0;
            pr2 := {| pr1 := B0; pr2 := {| pr1 := l; pr2 := g |} |}
          |} eqargs = map_In l g
        → let H := CoreTactics.block in map_In l g = map f0 l)


DEBUG:Going to execute:
simplify * || simplify ?; cbv beta