[siwei]

Commutativity of addition is not at all an obvious fact and I applaud your erstwhile employers for holding you to a higher standard of rigor.

[Osmigo]

Maybe it's just me, but I find it difficult to conceive of someone to whom, even after discussion and contemplation, (+4+3)=(+7) is not obvious. I suppose there are such people wandering the planet, however.

[sergiosgc]

Nitpick: the property observed here is associativity(sp?), not commutativity.