Goal:
    A : Type  B : Type  f0 : A → B
  ============================
   (let H := CoreTactics.block in
    map_In [] (λ (x : A) (_ : In x []), f0 x) = map f0 [])


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