Why does `filterBidirectional` exist, and why isn't it called `filter`?

[Ogi]

On Thursday, 9 March 2023 at 17:07:40 UTC, Paul Backus wrote:
>
> I think probably you have to go on a case-by-case basis. 
> `File.byLine`, for example, does not and should not precompute 
> `front`, because doing so is potentially very expensive.

And then ‘File.byLine’ ends up being const and now you can’t call 
‘front’ on it. Ouch.