\documentclass[12pt]{standalone}
\input{head}
\begin{document}
\begin{tikzpicture}\selectlanguage{english}
  \path (0,2) node(TTT){$\mathcal{S}$} (2,2) node(TTP){$\{\bigcup S\}_{S\in\mathcal{S}}$};
  \path (0,0) node(PTT){$\bigcup\mathcal{S}$} (2,0) node(T){$\bigcup\bigcup\mathcal{S}$};
  \tikzsquig (TTT) -- node[above]{$\power\mu_X$} (TTP);
  \tikzsquig (TTT) -- node[left]{$\mu_{\power X}$}(PTT);
  \tikzsquig (TTP) -- node[right]{$\mu_X$} (T);
  \tikzsquig (PTT) -- node[below]{$\mu_X$} (T);
\end{tikzpicture}
\end{document}
%%% Local Variables:
%%% mode: latex
%%% TeX-master: t
%%% End: