You mean like a `[<] or something? Perhaps, but then there would by extension be `[..<], `[<..] and `[<..<] too, and I think that would be unwieldy. I think it'd be unwieldy to have to write `[] and `[<] as two separate functions too, for that matter.
In that case I think it's better to simply change the `[] calling convention incompatibly. It's afterall something that #pike can cope quite well with.