It’s just a demo to “Hold It Against Me” — which, by the way, is a play on the lame “if I told you your body was beautiful …” pickup line — recorded by songwriter Bonnie McKee (she has credits on “California Gurls” and “Dynamite,” so, yeah, you know her), and when it reappears tomorrow, it will actually have Britney’s vocals on it. But, otherwise, one would assume, not too much will change. That means: A Euro-pop thump familiar to anyone who’s even minimally aware of the rise of Lady Gaga, a bluntly effective sing-along chorus, and all manners of tame sexual innuendo. Will this take off? Yeah, probably. But the more interesting question is how seamless Britney’s reimmersion will be, generally speaking. Remember when Christina Aguilera tried to come back and was swatted down in the court of Gaga? (Her movie got that Golden Globe nod, though!) This won’t be like that: Probably because Spears’s admirable-if-a-little-creepy-because-now-you-just-assume-everything-she-does-is-orchestrated-by-her-shadowy-overlords return from meltdown, the world seems to be always rooting for her. (Also, her Glee episode went over well, right?) But is Spears about to burst right back to megafame ubiquity, or will this be a new, more tamped-down phase of her career? Anyway: This is how your pop-music sausage is made.