ceylon.time.Period should be Invertible

[dlkw]

The API documentation doesn't mention much on negative Periods, and normalize() does not take them into account (which is probably another issue), but otherwise there doesn't seem to be a restriction concerning negative values for the individual parameters of this class.

So I suggest Period should satisfy Invertible to be able to subtract a Period instance from another.

[gavinking]

That sounds like a good idea. It already satisfies Summable and Scalable, so you can already write:

period1 + (-1 ** period2)

To do subtraction. Weird that you can't rewrite that as period1 - period2.

[gavinking]

Done. @luolong please check my work :)

[luolong]

Looks good to me.

[gavinking]

Great, thanks.