:db/tx (and Datomic's version, :datomic/tx) suffer from the same
ambiguities that [a v] lookup references do -- determining the type of
the result is context sensitive. (In this case, is :db/tx a reference
to the current transaction ID, or is it a valid keyword?) This commit
addresses the ambiguity by introducing a notion of a transaction
functions, and provides a little scaffolding for adding more (should
the need arise). I left the scaffolding in place rather than handling
just (transaction-tx) because I started trying to
implement (transaction-instant) as well, which is more difficult --
see the comments.
It's worth noting that this approach generalizes more or less directly
to ?input variables, since those can be eagerly bound like the
implemented transaction function (transaction-tx).
32ed56685e