[texhax] Puzzling (La)TeX output

[Barbara Beeton]

    I tripped over this and can't explain it; I hope someone here will  
    show me what I am missing:

    Using this source:

    =====
    \newcommand{\dx}{\rm dx}

    $\Phi(\dx_i)=dx_i$.
    =====

the braces get stripped off the expansion of the
command, so the \rm isn't scoped.  you could change
this to

  \newcommand{\dx}{{\rm dx}}

but it would be better to use the latex2e method of
indicating font changes in math by this:

  \newcommand{\dx}{\mathrm{dx}}

that definitively limits the scope of the roman to
its argument {dx} .
							-- bb