How come?

[metal56]

So that's three different uses of "come" without do-support, one current and two that were in use after do-support became pretty much generalised. As I said, interesting.

And I worry about my own meanderings on such things.

[Anuradha Chepur]

Idioms are lexical categories in the Chomskyan framework.
It seems to me that there are attempts to analyse idioms
in the GPSG and HPSG models.
However, one would have to spend some time studying those models
first.