AxisNames classThis is a wrapper for a list(), but the elements may only be singular
character(), ConstantAstNode() or other AstNode objects. (only 1 level
of nesting allowed, as its essentially another representation of a
OneSidedAstNode().
Note that when using c() on an AxisNames object, when the other object is
another AxisNames(), the elements of that will simply be appended to the
first object. To nest (in the case of a GroupAstNode), you must append
a list(AxisNames(...)) to the first object instead.
AxisNames(...)an AxisNames object, which is a list with a specific class
a list of elements or arbitrary number of elements