Currently, pattern-matching on 'Pair' is handled by treating Pair as a
record, which comes as slightly odd given that it isn't actually a
record and isn't user-defined. Thus now, every use of a record must
distinguish between Pairs and other kind of records -- which screams
for another variant constructor instead.
We cannot use `Tuple` either for this, because then we have no ways to
tell 2-tuples apart from pairs, which is the whole point here. So the
most sensical thing to do is to define a new pattern `Pair` which is
akin to tuples, but simpler since we know the number of elements and
it's always 2.
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