A namespace for nodes in the parse tree for selectors.
{CommaSequence} is the toplevel seelctor, representing a comma-separated sequence of {Sequence}s, such as `foo bar, baz bang`. {Sequence} is the next level, representing {SimpleSequence}s separated by combinators (e.g. descendant or child), such as `foo bar` or `foo > bar baz`. {SimpleSequence} is a sequence of selectors that all apply to a single element, such as `foo.bar[attr=val]`. Finally, {Simple} is the superclass of the simplest selectors, such as `.foo` or `bar`.