Allow predicate in Cols

[bkamins]

Fixes #2875

I was thinking that we might make it even more flexible and allow predicate to be mixed with other selectors in Cols, but for now I proposed a restrictive version. @nalimilan - do you think we should be more flexible?

[nalimilan]

Being restrictive sounds OK for now. Why does : need a special support? I would have thought it could be treated like any selector that can be included in Cols?

[bkamins]

Why does : need a special support?

Ah - you are right. : is not a function but a constructor so it is not a subtype of Function so special treatment is not needed. I will fix this.

[bkamins]

Ah - I was right originally Colon is a constructor but it is defined as:

struct Colon <: Function
end

so it is a Function. My original definition thus was required as otherwise Colon(:) treats : as a predicate. Things are tough.

[bkamins]

@nalimilan - this should be good to have a look at again.

[bkamins]

Thank you!