Example

% \f is defined as #1f(#2) using the macro
f\relax(x) = \int_{-\infty}^\infty
    f\hat\xi\,e^{2 \pi i \xi x}
    \,d\xi

% \f is defined as #1f(#2) using the macro
f\relax(x) = \int\_{-\infty}^\infty
    f\hat\xi\,e^{2 \pi i \xi x}
    \,d\xi