Enriching \to^*

Question

I am trying to enrich $\to^*$ with a $w$ above it, and the cleanest look I could get was with the {\smash {\overset w \rightarrow}}^* command. However, I believe this to be a pretty ugly solution, and \smash also smashes the space between two variables and the arrow itself (in for example $g {\smash {\overset w \rightarrow}}^* g'$. Is there a better solution that does not result in the w to be high above the arrow, or the * high above the arrow?

PS: How do you use mathmode or how can you display TeX commands? On other SE sites you can use $ $.

Answer 1

You have to put a "phantom" arrow; here \tostar is defined to have an optional argument, the letter that goes over the arrow (default w):

\documentclass{article}
\usepackage{amsmath}
\newcommand\tostar[1][w]{%
  \xrightarrow{#1}\mathrel{\vphantom{\to}^*}
}

\begin{document}
$a \tostar b$
\end{document}

We use the fact that TeX doesn't insert spaces between two relation symbols.