Drawing lozenges in Tikz

Question

I'm creating a simple flowchart using Tikz, and I'd really like the "decision" nodes to be lozenge-shaped (think "tilted parallelogram") rather than square, in order to fit text better.

Here's a minimal example, which is only the top part of the flow-chart and so doesn't illustrate the worst of the node in terms of text fit, but does show what I've got as opposed to what I would like, which is the "decision" node shape here:

\documentclass[a4paper]{article}

\usepackage{amsmath}

\usepackage{tikz}
\usetikzlibrary{positioning}
\usetikzlibrary{shapes.geometric}
\usetikzlibrary{arrows,automata}

% Define block styles
\tikzstyle{decision} = [diamond, draw, text width=10em, text badly centered, node distance=3cm, inner sep=0pt]
\tikzstyle{line} = [draw, -latex']
\tikzstyle{cloud} = [draw, ellipse, node distance=3cm]

\begin{document}

\begin{figure}
\begin{center}
\begin{tikzpicture}
% Nodes
  \node[cloud] (a) {$p_n$ is proposed.};
  \node[decision, below of=a] (b) {Has a positum $\phi$ been admitted?};
  \node[decision, below left of=b] (c) {Is $p_n$ true?};
  \node[cloud, below right of=b] (g) {Nothing happens.};

% Edges
  \path [line] (a) -- (b);
  \path [line] (b) -- node {no}(g);
  \path [line] (b) -- node {yes}(c);
\end{tikzpicture}
\end{center}
\end{figure}

\end{document}