Goal:
  
  A : Type
  B : Type
  C : Type
  D : Type
  R : C → D → Type
  f : A → C
  g : B → D
  H : All2 R [] []
  H0 := {| pr1 := {| pr1 := []; pr2 := [] |}; pr2 := H |}
     : sigma
         (λ index : sigma (λ _ : list C, list D),
            All2 R (pr1 index) (pr2 index))
  H1 := {| pr1 := []; pr2 := [] |} : sigma (λ _ : list C, list D)
  H2 := H
     : (λ index : sigma (λ _ : list C, list D),
          All2 R (pr1 index) (pr2 index)) H1
  ============================
   (H = H2
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in
      All2 (λ (x : A) (y : B), R (f x) (g y)) [] [])


DEBUG:Going to execute:
<coq-core.plugins.ltac::clearbody@0> $1