[antlr-interest] Data value is field length

[joel at mentics.com]

So, if I have a binary field, the first byte of which indicates how
long the field is, is there a way to do this in ANTLR?

The Lexer would have to get the first byte, look at its value, read
that many more bytes, and that would be the end of that field.

Any ideas on how this might best be done in ANTLR?