"Concurrent" operations are not necessarily actually concurrent, so
"divergent" seems like a better name. And "reconcile" seems like a
better term for merging them, though we also sometimes use "merge".
A possible use case is when doing some archaeology around a certain operation.
The current implementation is quadratic if + is repeated. Suppose op_id is
usually close to the current op heads, I think it'll practically work better
than building a reverse lookup table.
