I considered <- (along with a lot of other things), but rejected it because the left-associativity of it seemed counter-intuitive, it didn't stand out as much as slashes, and I want it to stand out because it's inconsistent with symbol syntax, it's confusing with -> which conventionally means type-conversion (and which doesn't break up symbols), and it's three times as much effort to type as /.

Slashes are fairly common in symbols, but not at the beginning (except for / itself), so it's unambiguously parseable, though maybe it won't work with this system, now that I think about it. I don't see how to prevent embedded slashes from meaning anything special without the first slash. I.e., I want foo/bar/nitz to be one symbol, but /foo/bar/nitz to be a symbol and two msg calls. Maybe that's too hairy, but I'd hate to break things like w/uniq.