Goal:
    A : Type  P : list A → A → Type  x : A  a1 : list A  a : All_fold P a1
  ============================
   ([x] = a1
    → let H := CoreTactics.block in let H0 := CoreTactics.block in P [] x)


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