Operators and commutation

Tom, can you refresh my memory on the preferred way to define
"commutative operators" for operators with mixed input types? For
example, I want to define a new operator to add an interval to a time.
Do I need to fully implement the commutative function which adds a time
to an interval, or is there another way?

I used to have a cheat interpretation of commutation during operator
matching in the parser (which allowed a mixed-type operator to refer to
itself as its commutator, and the parser would then flip the arguments
around to match up), but I recall that you took this out to reinforce
the purity of the interpretation of commutation in the table flags. So
what is the best way to do this now?

- Thomas