Way back last summer, Brooklyn DA Joe Hynes helped end Mark Green's campaign for Attorney General by leaking to the Post word of a revived investigation into Green's 2001 campaign. People around Green were convinced that it was a Cuomo hit, recalling that Mario gave Hynes his start in politics.

People around Cuomo responded that Hynes and Cuomo fell out over an ancient slight at the 1994 state Democratic convention, and so there was no reason to think Hynes would do Cuomo a favor.

That latter theory made sense until yesterday, when Cuomo put out a press release announcing that none other than Joe Hynes would be leading a review of his Medicaid unit. (Unclear who's doing whom a favor there -- some of both, it seems, giivng Hynes some publicity and getting the Post off Cuomo's back.)

In any case, this being January 9, they appear not to be observing the traditional waiting period between your secret ally's playing a rather dirty trick and your publicly embracing him.

As for the Green investigation, it appears to have vanished again, oddly, soon after the primary. Hynes's spokesman, Jerry Schmetterer, didn't return repeated calls about this in November. Witnesses who were re-summoned before a new grand jury never heard back. And any 5-year statute of limitations has now expired.

So what was that investigation? And did it have any purpose other than killing Green? It certainly didn't have any other effect.