bold math fonts for matrices

[Tuukka Toivonen]

Another font problem.

Normal variables are written in italics. All right.

Some things -- like functions -- are written in normal font. I can
get that using Alt-M-M in math mode. All right.

But how can I get bold font for things like matrices?

Or more generally: how to change the font type and attributes in math
mode? Is it even possible?