The term “functor” is utterly alien to the programming mainstream, where “generic” is the usual term, so Mythryl replaces the functor keyword with generic package to maximize effectiveness of C-trained intuition.
By convention, Mythryl names generics with identifiers ending in _g, but this convention is not compiler-enforced:
generic package red_black_map_g (k: Key): Map where key == k
{
...
};
(For complete source to the above example see src/lib/src/red-black-map-g.pkg.)