# Can a command typeset its argument without letting latex take it into account for bounding box?

Quite often, especially when preparing slides, vertical space is precious. However, there is no automatic equivalent of "vertical kerning of lines", by which you would reduce or even have a negative line spacing when the highest elements of the lower line are on the right hand side while the upper line occupies only the left hand side.

A small example:

\begin{minipage}{7cm}
Equation:
$$a+b+c+d=\overbrace{x+y+z}^{\text{unknown}}$$
\end{minipage}


With commands such as \hphantom, you can have Latex not typeset the argument and reserve the space in bounding box. Can one do the reverse, display the overbrace above without updating the bounding box of the equation?