@johoelzl I'm using `abstract` tactic because instances are
automatically marked as [reducible], and they will be unfolded when
solving unification constraints. This cannot be avoided since we need to
solve unification constraints such as
int.has_add =?= comm_ring.to_has_add int.comm_ring
The `abstract tac` tactic creates an auxiliary lemma to store the proof
generated by `tac`. If we use `print int.comm_ring` we can see that
the definition is much smaller. The proofs are irrelevant. So, this has
no drawbacks, and gives us a good performance boost.
7ac6b14d2a