Set: pp::colors
  Set: pp::unicode
  Assumed: vec
  Assumed: concat
  Assumed: concat_assoc
  Assumed: empty
  Assumed: concat_empty
  Assumed: n
  Assumed: v
  Assumed: w
v ; w ; empty ; (v ; empty)
===>
v ; (w ; v)
htrans (htrans (hcongr (hcongr (hcongr (hcongr (hrefl @concat) (to_heq (Nat::add_zeror (n + n))))
                                       (to_heq (Nat::add_zeror n)))
                               (htrans (to_heq (concat_empty (v ; w)))
                                       (cast_heq (congr2 vec (symm (Nat::add_zeror (n + n)))) (v ; w))))
                       (htrans (to_heq (concat_empty v)) (cast_heq (congr2 vec (symm (Nat::add_zeror n))) v)))
               (to_heq (concat_assoc v w v)))
       (cast_heq (congr2 vec (symm (Nat::add_assoc n n n))) (v ; (w ; v)))