\begin{tikzpicture}[>=stealth']
  \matrix[column 1/.style={nodes={text width=18em,text ragged,font=\scriptsize}}] at (-0.5,-1)
  {
    \node {$\mathsf{simple}\leq \mathsf{pos1}$}; \\
    \node {$\left\{\mathsf{plural1, simple}\right\}\leq \mathsf{pos1}$}; \\
    \node {$\mathsf{simple}\leq \mathsf{pos2}$}; \\
    \node {$\left\{\mathsf{plural2, simple}\right\}\leq \mathsf{pos2}$}; \\
  };
  \node (ened) at (-0.5,-3) {$\left\{\mathsf{plural1, simple}\right\}$};
  \node (ened2) at (1,0) {$\mathsf{pos1}$};
  \node (ded) at (2.5,-3) {$\mathsf{simple}$};
  \node (dened) at (4,0) {$\mathsf{pos2}$};
  \node (dened2) at (5.5,-3) {$\left\{\mathsf{plural2, simple}\right\}$};

  \draw[<-] (ened2) to (ded);
  \draw[<-] (ened2) to (ened);
  \draw[<-] (dened) to (dened2);
  \draw[<-] (dened) to (ded);
\end{tikzpicture}

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