The implications of that sentence—assuming, first of all, that it is true and that the Doctor is not making things up to try and keep her in place—are so hair-raising that she can't even bring herself to suggest that perhaps she'll test this rule on his skinny arse right now.

Instead she says, "I see. And are there any other arrant impossibilities I ought to know about besides that? And to which part of this place have the two of you been banished?"