This differs from [the original][GOO] in expecting an extra layer of
parentheses. I find it easier to put the extra parentheses in than to
remember to leave them out. Doing it this way also lets completion
work.

Of course, the extra parentheses make it longer, but the point of
positional lambdas isn't to save typing: it's to save the mental
effort of giving things *names* when all we are interested in is the
*shape* of the code.

[GOO]: http://people.csail.mit.edu/jrb/goo/manual.46/goomanual_15.html#17
