🚀 go-pugleaf

Hey people:

when I write an equation such as:

\begin{equation}
f \frac{\partial f}{\partial \mu} = \frac{\rm{d} f}{\rm{d} \mu}
\end{equation}

the f's on the left hand side of the equation are typeset as a script form
of f (which is what I want), whereas those on the right hand side are roman
f's even though they are not contained in the \rm{}.

How do I control this?  Is there a \script{f} or similar available?

Thanks in advance,

Geoff.