[selectors] Previous-sibling combinator?

The existence of the following-sibling combinator, +, begs the question:
why doesn't a previous-sibling combinator exist? I can't imagine that it's
any harder to implement, and I can't come up with any semantic objection -
in fact, the lack of symmetry would be a semantic argument for it.

If the above seems at all reasonable, the symbol "-" would make sense for
the job.

Clive Chan