Utility Keys

Utility key codes are used to fill space:

Key	Aliases	Description
OPAQUE	X, XX, XXX, …	Ignore this key; a no-op.
TRANSPARENT	_, __, ___, …	Check the key in the next layer.

OPAQUE

A black hole that swallows events but has no side effects. Useful when composing layers.

TRANSPARENT

A key that allows and event to go through to the next lower layer. Useful when composing layers.