reverse functions for sin, cos, arc

[janwirth]

am i missing arcsin, arccos and arctan somewhere?

[kizu]

You can use math bif, like this: math(0.5, 'acos')

But we could add them as their own bifs, with some helpers for degrees and all that stuff normal sin/cos bifs have. If someone would send us a PR for that, it would be really nice.

As we have already an issue about this (#1567), I'll close this one as a duplicate.