I worry a bit about the `@each` form.  `sumBy("foo.@each.bar")` maybe makes it seem that brace expansion could be used here.  The semantics would not be obvious although presumably it would be to sum both.

Personally, I'd rather not support the `@each` form than to have it (from the user's POV, weirdly) not support brace expansion.

What do you think?  Am I being overly paranoid about consistency?