Goal:
    A : Type  B : Type  R : A → B → Type  x : A  y : list B  a : All2 R [x] y
  ============================
   (∑ y' : B, y = [y'] × R x y')


DEBUG:Going to execute:
intro_block_id H || intro_block H
Level 0: Backtrace:

No quantified hypothesis named a in current goal even after head-reduction.
Level 0: Backtrace:

No quantified hypothesis named a in current goal even after head-reduction.