[Python-3000] sets in P3K?

[Greg Wilson]

> A set with no elements is:
>    set()

I prefer Guido's earlier suggestion of {/} for the empty set (assuming no
parsing ambiguities?):

a) requiring 'set()' for empty sets is inconsistent with (), [], {} for
   empty collections of other kinds

b) {/} kind of looks like phi (circle with a slash through it, to quote
   my engineering students ;-), which some of the books on my shelves use
   for empty set.

Greg