Goal:
  
  A : Type
  B : Type
  R : A → B → Type
  x : A
  y : list B
  a1' : list A
  a1 : list B
  a : All2 R (pr1 {| pr1 := a1'; pr2 := a1 |})
        (pr2 {| pr1 := a1'; pr2 := a1 |})
  ============================
   ({| pr1 := [x]; pr2 := y |} = {| pr1 := a1'; pr2 := a1 |}
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in ∑ y' : B, y = [y'] × R x y')


DEBUG:Going to execute:
hnf in id;
            match type of id with
            | sigma _ =>
                let xn := fresh id "'" in
                destruct id as [xn id]; DepElim.decompose_exists xn id;
                 cbv beta iota delta [TODO projection TODO projection]
                  in id, id'; DepElim.decompose_exists id id'
            | _ =>
                cbv beta iota delta [TODO projection TODO projection]
                 in id, id'
            end