Goal:
  
  A : Type
  B : Type
  R : A → B → Type
  x : A
  y : list B
  a1' : list A
  a1 : list B
  a : All2 R a1' 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:
cbv beta iota delta [TODO projection TODO projection] in id, id'
DEBUG LTAC Evaluated term: a1
DEBUG LTAC Evaluated term: (list B)
Level 4: type of id
 has value 
(list B)