[PATCH] revset: add experimental set subscript operator

[Jun Wu]

Excerpts from Sean Farley's message of 2017-07-01 16:33:47 -0700:
> > Yep. That's why I slightly prefer not using "{}".
> >
> >   $ hg log -T '{revset(".{0}")}\n'
> >                          ^^^
> >                    this is not a revset operator, but expanded to ''
> 
> I like separating {} and [] so that foo{4}[1] is the 4th level of
> descendants of foo and the [1] would be the index in that list (ordered
> by rev num?).

Could it be foo[+4][1] meaning what you want? An explicit "+" means it's an
offset, the "1" without a "+" means it's a normal index.

> > That's mpm's point why he chose {} over [].
> >
> > http://markmail.org/message/sjnnwa43s4eksu62
> >
> > FWIW, the use of [] can be justified by considering x[n] as a sort of
> > a pointer arithmetic in DAG structure.
> >
> > We need APL keyboard.
> 
> So, so true.