% Logic diagram
% Author: Stefan Kottwitz
\documentclass{article}
\usepackage{tikz}
%%%<
\usepackage{verbatim}
\usepackage[active,tightpage]{preview}
\PreviewEnvironment{tikzpicture}
\setlength\PreviewBorder{5pt}%
%%%>
\begin{comment}
:Title: Logic diagram
:Tags: Matrices
:Author: Stefan Kottwitz
:Slug: logic-matrix

Math formulas and arrows with TikZ
http://texblog.net/latex-archive/maths/logic-tikz/

In the Art of Problem Solving Forum http://www.artofproblemsolving.com/Forum/viewtopic.php?p=1359679#1359679
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.

This example uses TikZ for arrows and its matrix library for math nodes.
Some nodes are connected by -| or |- path construction operations.
\end{comment}
\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}