[E-Lang] Immutable map operations

[Dean Tribble]

I have a very similar reaction to the |[] syntax.  It's all very 
principled, but when I step back, I can't imagine using it.  Thank you for 
articulating why :-)

> > >        str |= [ 'a' ]
> > >
>...
>Its the extra set of brackets that bother me.
>
>All I wanted to do was add a single element to the collection. This ...
>
>The "| []" operator also reads like I am creating a new set and
>computing the union. That may very well be a compatible interpretation
>of the operation, but it's not the one I am thinking about when I want
>to insert a single element.

This last point is extremely important, and hadn't been called out 
before.  This has been the real source of my discomfort with the 
solution.  Adding a single element is so basic that any distance from that 
is dissonant.

>The combination of two operators "|" and "[]" suggest that there are
>two operations being done on this line of code and neither of them
>immediately seem to have anything to do with "insert a single
>element".

[I elided the question for MarkM about what the syntax means for a 
collection multiple arguments because there are several plausible answers, 
and I don't think they change the tenor of the proposal]