\begin{tikzpicture}[>=stealth', line width=.15em]
  \node (stem) at (-5,0) {\it stem};
  \node (stemm) at (-1,0) {{\it stem}\bf{m}};
  \node (ndstemm) at (4,2) {\bf{ned}{\it stem}\bf{m}};
  \node (nstemm) at (4,-2) {\bf{n}{\it stem}\bf{m}};
  \node (ndstem) at (-7,-3) {\bf{ned}{\it stem}};
  \node (nstem) at (-2,-3) {\bf{n}{\it stem}};

  \draw[->,dashed] (stem) to [out = 35, in = 170] node [fill=white] {\footnotesize $ned\circ m$} (ndstemm);
  \draw[->,dashed] (stem) to [out = -35, in = 190] node [fill=white] {\footnotesize $n\circ m$} (nstemm);

  \draw[->] (stem) to node [fill=white] {\footnotesize $m$} (stemm);
  \draw[->] (stem.south) to node [fill=white] {\footnotesize $ned$} (ndstem);
  \draw[->] (stem.south) to node [fill=white] {\footnotesize $n$} (nstem);
  \draw[->] (stemm) to node [fill=white] {\footnotesize $ned$} (ndstemm);
  \draw[->] (stemm) to node [fill=white] {\footnotesize $n$} (nstemm);
\end{tikzpicture}

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