Anis
Anis
Look at the Animation reference pages before reading these docs.
This Anis class serves the same role that Group does for Sprites.
This class is used internally to create `sprite.anis`
and `group.anis`.
In instances of this class, the keys are animation names,
values are Ani objects.
Because users only expect instances of this class to contain
animation names as keys, it uses an internal private object
`#_` to store animation properties. Getters and setters are used to
access the private properties, enabling dynamic inheritance.