set phrase

Bally remarks in passing, as Hall does not, that the inversion in toujours est-il que is part of a set phrase and hence invariable.

If it proves clearly unfeasible to make the audience laugh at a thin and far-fetched joke, it is always better to change the way the joke works . . . for instance, a pun based on the speaker's taking literally some set phrase or metaphor with a pun based on phonetic similarity.