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We do not want to generate distinct liftings for every single
partial application of a function or predicate symbol. Canonical
closures have an easily recognizable shape, thus we can detect
them and replace them with a unique constant "f_closure".

Currently, the builtin theory why3.HighOrd (or just HighOrd) must
be explicitly "use"-d. However, the type (HighOrd.func 'a 'b) can
be written ('a -> 'b), and the type (HighOrd.pred 'a) can be written
('a -> bool), and the application operation (HighOrd.(@)) can be
written as the usual juxtaposition. Thus, normally, you do not have
to write the qualifiers. The builtin theory why3.Bool (or just Bool)
is needed for "bool". The names "HighOrd", "func", "pred", and "(@)"
are not yet fixed and may change.

"eliminate_epsilon" tries to be smart when a lambda (or some other
comprehension form) occurs under equality or at the top of a definition.
We could go even further and replace (\ x . t) s with t[x <- s], without
lifting the lambda. I'm not sure it's worth it: we rarely write redexes
manually. They can and will appear through inlining, though.

Anyone who wants to construct epsilon-terms directly using the API
should remember that these are not Hilbert's epsilons: by writing
an epsilon term, you postulate the existence (though not necessarily
uniqueness) of the described object, and "eliminate_epsilon" will
happily convert it to an axiom expressing this existence. We only
use epsilons to write comprehensions whose soundness is guaranteed
by a background theory, e.g. lambda-calculus.