To clarify my review manager note:
toggled() is a reimplementation of ! as a function, and as such is probably on the commonly rejected proposals list. I say probably because the reasoning for rejecting that particular variant isn't listed, but I expect we'd consider it covered by it.
Whether to add toggled() or not would also be a separate proposal that doesn't affect whether or not a mutating toggle() should be accepted, so if it weren't on the commonly-rejected list, it should be raised as a new pitch rather than discussed in this review.