2011/10/12 Lars Hellström <[log in to unmask]>:
>  True = \lambda xy . x
>  False = \lambda xy . y
>
> These may look strange, but turn out to be two objects that are very
> familiar to us: \use_i:nn and \use_ii:nn respectively (or \@firstoftwo and
> \@secondoftwo, for those who still think in 2e terms (like me)).

This is in fact used by etoolbox's toggles.