[TeMPOraL]

> At this point you might as well do `m1 == m2.convert_to(m1.currency)`, because "HaveSameWorth" might mean many different things too.

I personally hate that last style because it's obvious that the "HaveSameWorth" relation is intended to be symmetric, and by writing it like m1 == m2.convert(...) you're prefering one side over the other. It looks bad for me :).

Also, in case of real-life objects it makes sense to spell out what do you mean by 'equality' (or 'equivalency'), and leave the default implementation to represent the philosophical concepts of "the same" and "equivalent to".

[cdcarter]

But you almost certainly are logically preferring one side over the other! You do usdValue == cadValye.convert_to(usdValue.curr) because one of those currencies is the one your transaction is working with (in this case, USD is your goal).