[texhax] math bold not extended?

[Michael Barr]

I am trying to define a command \mathbi that will produce an ordinary bold 
math character.  The command
\DeclareMathAlphabet{\mathbi}{OT1}{cmr}{bx}{it}
makes an extended bold character.  From the Companion, I glean the fact 
that \mathbold (or \mathversion{bold}) changes the math alphabet to such a 
font, but I cannot find how to define \mathbi as I want it.

Michael Barr