Why aren't colons optional?

[Courageous]

>   In this case, I'd (depending on the parser technology used, I like ANTLR)

I like ANTLR, too. I was just curious how it was your were going
to go about creating context-dependency. This isn't impossible in
limited cases where you remove certain types from consideration in
segments of the parse tree, but it does introduce complexities into
parsing that some parser authors would rather avoid, wouldn't you
say? Or am I going down the wrong track here?

I'll consider that possibly I am, as I haven't really considered
what grammatical constructs we're really selecting between in any
detail.

C//