my_bld.splice (my_bld.begin (), buildings_);
single_skyline (Building (b, horizon_padding, a, sky_), b[a][LEFT], horizon_padding, &other_bld);
internal_merge_skyline (&other_bld, &my_bld, &buildings_);
my_bld.splice (my_bld.begin (), buildings_);
single_skyline (Building (b, horizon_padding, a, sky_), b[a][LEFT], horizon_padding, &other_bld);
internal_merge_skyline (&other_bld, &my_bld, &buildings_);