Math formulas and arrows with TikZ

In the Art of Problem Solving Forum somebody asked for help in creating a construction for proofs and demonstrations in Logic by LaTeX commands. Math expressions should be aligned, some connected by arrows going vertically and horizontally.
Regarding arrows I would use tikz and also its matrix library for math nodes. The nodes could be connected by -| or |- path construction operations, like

\draw (node1) -| (node2) |- (node3);

For creation of the example given in the thread on AoPS I’ve used a matrix of math nodes, some empty nodes for alignment and |- -| arrows, here’s the compilable example code:

\documentclass{article} \usepackage{tikz} \usetikzlibrary{matrix} \begin{document} \begin{tikzpicture}[every node/.style={anchor=west}] \matrix (m) [matrix of math nodes, nodes in empty cells]{ \quad & 1.\quad (x)(Q\supset Fx) & \\ & 2.\quad Q\supset Fx & \textrm{I,\textbf{UI}} & \\ & 3.\quad Q \\ & 4.\quad Fx & 2, 3, \textrm{ M.P.}\\ & 5.\quad (x)Fx & 4, \textrm{ \textbf{UG}} \\ & 6.\quad Q\supset(x)Fx & 3-5, \textrm{ C.P.} \\ & 7.\quad \parbox[t]{2.9cm}{% $(x)(Q\supset(x)Fx)\supset$\\ $[Q\supset(x)Fx]$} & 1-6, \textrm{ C.P.}\\}; \draw[-stealth] (m-7-2.north east) -| (m-1-1.west) |- (m-1-2); \draw[-stealth] (m-6-2.north east) -| (m-3-1.east) |- (m-3-2); \end{tikzpicture} \end{document}

Output:

19. January 2009 by stefan
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