> Same here for CHICKEN, but I'm a bit hesitant to go in and complicate
> the numerical syntax of Scheme.  We just got rid of the ridiculous hash
> syntax (which _really_ complicated Scheme numerical syntax)!

I'm sympathetic to that - simplicity is good. I was thinking it would be
best to only allow it between digits, and deny it at the beginning or
end of a token. And perhaps also to disallow more than one consecutive
underscore. So like this:

   1_000_000    ; good
   1_000_000_   ; bad
   _1_000_000   ; bad
   _10000000    ; bad
   10000000_    ; bad
   1__000_000   ; maybe bad

If underscores are allowed at the beginning of a token, then there is
more possibility of conflict -- e.g. in C you can write a function named
'_123'.