I'd just like to interject for a moment. What you're referring to as air, is in fact,
!#existing, or as I've recently taken to calling it, air + cave_air + void_air. air is
not a non-existing mask unto itself, but rather another free component of a fully functioning mask
made useful by the WorldEdit parser, Minecraft and vital naming components comprising a full mask
as defined by EngineHub.
Many WorldEdit users run a modified version of the !#existing mask every
day, without realizing it. Through a peculiar turn of events, the version of the mask which is
widely used today is often called air, and many of its users are not aware that it is basically
the !#existing mask, developed by the WorldEdit team.
There really is an air, and these people
are using it, but it is just a part of the mask they should use. air is the old block: the block
in the mask that commonly appears above-ground. The air block is an essential part of the mask,
but confusing by itself; it can only function properly in the context of a complete mask. air is
normally used to mean !#existing: the whole mask is basically air with cave_air and
void_air added, or !#existing. All the so-called air masks should really be !#existing.