[PRL] Tex Question: Bold font for Greek letters

[Kenichi Asai]

Hi,

Does anyone know how to write Greek letters in bold font or any other
forts (in math mode)?  I tried $\mathbf{\lambda}$ but it seems it does
not work.  I want to express lambda terms that use \lambda, @, \xi,
\langle, and \rangle in two different fonts (to distinguish the object
language from the meta language).

Thanks in advance.

# It might be a bad time to ask questions.  All the books are in the
# packages...

-- 
Kenichi