parsing format/{1,2,3} format strings

Does anyone have code for parsing format strings used by format/{1,2,3}
predicates?  I'm imagining something roughly like this:

?- parse_format("Total: ~2:d~n", X).
X = [string("Total: "), tilde(2, :, d), tilde(default, '', n)].

It'd be great if the predicate worked in both directions (for generating
format strings too), but I really only need the parser at the moment.

Thanks.

-- 
Michael
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