Another category of presupposition, “Contrary to Expectation” is using the word “should” (or a similar conditional) to express that the outcome mentioned is not what we expect.

If you should happen to not get this the first time, you can join the club. It took me several goes to get it.

If you do happen to lose your place in line, I’ll buy your popcorn for you.

By the way - I failed to label the last language pattern I had up here as being a presupposition. For those of you keeping track, Counterfactual Conditional Clauses from the Jan. 9th post here at dougobriensblog.com is also a Reverse Meta Model: Presupposition.