Add bg'

[jeffreyrosenbluth]

I often have need of a convenience function like this in particular when making gallery images. Like vsep and hsep it is primarily to save a bit of boilerplate typing.

[byorgey]

My only other quibble is that usually primed versions of things take an options record; here it's just an extra Double. So perhaps it would be better to give it a more descriptive name? I don't know, just a thought. In the end I don't really care that much since bg is not a very central function.

[jeffreyrosenbluth]

I agree, it's probably a good idea to reserve primed versions for functions taking and options record. I'll see if i can come up with a better name.