[texhax] some problems about \prevgraf and \toks

[Karl Berry]

    >So, my question is how can I refer to the current line number or at least 
    >how can I know that whether TeX has already entered a new line or not?

As Ian explained, there's no obvious way to do this because of the way
TeX works.  It's unfortunate.

However, see the FAQ entry for some packages that do their best to make
it possible:
http://www.tex.ac.uk/cgi-bin/texfaq2html?label=linenos

Regards,
karl