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Turns out, we never use this requirement. The only place where it may
come in relevant is the free theorems. But here non-injectivity turns
out to be reasoning about multiple get functions that are selected by
the additional index each individually satisfying the free theorem and
thus commonly satisfying it as well.
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Most of the became unused by using the convenience functions introduced
in the parent commit.
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These two features heavily interconnect. As such it makes sense to
integrate them properly. This non-trivial merge does that work. Compared
to feature-partial-getlen a few definitions moved from FreeTheorems.agda
to GetTypes.agda. Many types changed compared to both branches.
Conflicts:
BFF.agda
Bidir.agda
FreeTheorems.agda
Precond.agda
conflict in GetTypes.agda not detected by merge
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This allows passing both getlen and get as a single parameter. It also
allows to make the free theorem a prerequisite instead of a postulate.
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The representation chosen is to give both an injection gl₁ and a
function gl₂ (formerly getlen), such that by choosing a non-identity for
gl₁ partiality of getlen can be expressed. An alternative would have
been to allow getlen to return a Maybe ℕ and have get return
maybe (Vec A) ⊤ (getlen n)
thus sending all inputs for which getlen yields nothing to tt. It seems
that while there is no way to obtain a such a getlen predicate from an
arbitrary index Setoid I, it should be possible to manufacture a Setoid
from a predicate. Thanks to Stefan Mehner for the insightful discussion.
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This should make it easier to see what is assumed.
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