Goal:
    A : Type  B : Type  R : A → B → Type  x : A  y0 : B  r : R x y0
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in ∑ y' : B, [y0] = [y'] × R x y')


DEBUG:Going to execute:
match goal with
           | hyp:context [ CoreTactics.block ]
             |- _ =>
                 cbn beta in hyp; eqns_specialize_eqs_block hyp;
                  cbn[eq_rect_r eq_rect] in hyp
           end