[texhax] Change \item behavior, to append argument to default list label, instead of replacing it.

[Niall Mansfield]

I want to change the behavior of \item within a list, so that any \item 
argument is *appended* to the default list label ("ABC", say) instead of 
replacing it.

The following is what I would like.  Let's say my "foo" environment 
typesets a list with "ABC" instead of bullets.  Then:

  \begin{foo}
  \item the quick
  \item[BONGO] brown fox
  \item jumps
  \item[bar] over the dog
  \end{foo}

should produce:

  ABC the quick
  ABC BONGO broxw fox
  ABC jumps
  ABC bar over the dog

Can I do this?

    Thanks,
    Niall