Vertical alignment through all document

Question

I am processing a text, which is the specification for my mathematical package. The typical style for the specification looks as follows:

I can align expressions only in a single environment, using align or array and so. But I want, that all examples from presented image, will be aligned with respect to \to symbol through all document. Is there any way to do so?

The code of the presented image (sorry that it rather complicated):

\documentclass[a4paper,titlepage,11pt,oneside]{book}

\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{amsthm}
\usepackage{latexsym}
\usepackage{array}
\usepackage{xargs}
\usepackage{xparse}
\usepackage{xcolor}
\usepackage{empheq}
\usepackage{courier}

\newcommand{\mkey}[1]{{\mbox{\small\ttfamily #1}}}
\newcommand{\mobj}[2]{\mbox{\ttfamily\small #1}\left [\,#2\,\right]}


\definecolor{boxcolor}{rgb}{.95, .95, 1}

\newsavebox{\mysaveboxM} % M for math
\newcommand*\MetaBox[2][Example]{%
 \sbox{\mysaveboxM}{#2}%
 \sbox{\mysaveboxM}{%
  \parbox[b][\ht\mysaveboxM][b]{%
  \wd\mysaveboxM}{#2}%
 }%
 \sbox{\mysaveboxM}{%
 \fcolorbox{black}{boxcolor}{%
  \makebox[\linewidth-4em]{\usebox{\mysaveboxM}}%
  }%
 }%
 \usebox{\mysaveboxM}%
}

\NewDocumentEnvironment{meta}{O{equation*}O{}}{%
 \setkeys{EmphEqEnv}{#1}%
 \setkeys{EmphEqOpt}{box=\MetaBox,#2}%
 \EmphEqMainEnv}%
 {\endEmphEqMainEnv}


\begin{document}
\subsection{Product}
....
\subsubsection{Indices}
\begin{meta}
\begin{array}{rcl}
\mobj{}{H^{kj}M_jQ_k{}^{\mu}{}_{qa}}\mkey{.indices}&\to&
\mobj{UnorderedIndices}{{}^{kj}{}_{jk}{}^{\mu}{}_{qa}}
\end{array}
\end{meta}
The interesting point introduced here is a special type of product
indices. We will discuss it in section. The general idea here is that  
 from the mathematical point of view the order of resulting product indices          
is undefined.
\subsubsection{List access operations}
\begin{meta}
\begin{array}{rcl}
\mobj{}{V_{\alpha\beta}M_\sigma
Ric^{\kappa\delta}\,\delta^\alpha{}_{\kappa}}\mkey{.size()}&\to&4\\[12pt]
\mobj{}{H^{kj}M_jQ_k{}^{\mu}{}_{qa}}\mkey{.elements}&\to&
\mobj{List}{H^{kj},\,M_j,\,Q_k{}^{\mu}{}_{qa}}
\end{array}
\end{meta}
One of the important (and really hard) decisions we've made is to
consider only commutative theories. So, while processing products, the order   
of elements is not taken into account! However, there are some tricks to       
work with non-commutative theories.\newline Indeed, you can take an element 
by index:
\begin{meta}
\begin{array}{rcl}
\mobj{}{H^{kj}M_jQ_k{}^{\mu}{}_{qa}}\mkey{.get(2)}&\to&
\mobj{SimpleTensor}{Q_k{}^\mu{}_{qa}}
\end{array}
\end{meta}

\end{document} 

Answer 1

This solution uses TikZ to find where the symbol is and then cuts away the part left of it from the bounding box. Then, the \symbolOffset specifies how far to move the symbol from the left margin. The offset will be the across the entire document (provided \symbolOffset isn't set to a new value somewhere). Since the environment meta automatically centers its content, the array is wrapped by a minipage to get left alignment again (otherwise the offset doesn't work). The document must be compiled twice for this to work.

The downside of this solution is that you have to adapt the offset manually such that no part overflows.

I've incorporated my solution into your example:

\documentclass[a4paper,titlepage,11pt,oneside]{book}

\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{amsthm}
\usepackage{latexsym}
\usepackage{array}
\usepackage{xargs}
\usepackage{xparse}
\usepackage{xcolor}
\usepackage{empheq}
\usepackage{courier}

\newcommand{\mkey}[1]{{\mbox{\small\ttfamily #1}}}
\newcommand{\mobj}[2]{\mbox{\ttfamily\small #1}\left [\,#2\,\right]}

\definecolor{boxcolor}{rgb}{.95, .95, 1}

\newsavebox{\mysaveboxM} % M for math
\newcommand*\MetaBox[2][Example]{%
 \sbox{\mysaveboxM}{#2}%
 \sbox{\mysaveboxM}{%
  \parbox[b][\ht\mysaveboxM][b]{%
  \wd\mysaveboxM}{#2}%
 }%
 \sbox{\mysaveboxM}{%
 \fcolorbox{black}{boxcolor}{%
  \makebox[\linewidth-4em]{\usebox{\mysaveboxM}}%
  }%
 }%
 \usebox{\mysaveboxM}%
}

\NewDocumentEnvironment{meta}{O{equation*}O{}}{%
 \setkeys{EmphEqEnv}{#1}%
 \setkeys{EmphEqOpt}{box=\MetaBox,#2}%
 \EmphEqMainEnv}%
 {\endEmphEqMainEnv}



% --- NEW STUFF --------------------
\usepackage{tikz}
\usetikzlibrary{calc}

\newlength{\symbolOffset}
\setlength{\symbolOffset}{6.2cm} % Distance between the left margin and the
                                 % symbol
\newcommand{\thesymbol}{$\to$} % Symbol
\newcommand{\alignSymbol}{%
  \begin{tikzpicture}%
    \node [inner sep=0pt] (symbol) {\thesymbol};
  \end{tikzpicture}%
}

\newcommand{\alignArray}[1]{%
  \begin{minipage}{\textwidth}%
    \hspace*{\symbolOffset}%
    \begin{tikzpicture}[remember picture]%
      \node (array) [inner sep=0pt] {${#1}$};
      \pgfresetboundingbox
      \path [use as bounding box]
        (symbol |- array.north) rectangle (array.south east);
    \end{tikzpicture}%
  \end{minipage}%
}
% --- END NEW STUFF ----------------



\begin{document}
\subsection{Product}
....
\subsubsection{Indices}
\begin{meta}
  \alignArray{
    \begin{array}{rcl}
      \mobj{}{H^{kj}M_jQ_k{}^{\mu}{}_{qa}}\mkey{.indices}&\alignSymbol&
      \mobj{UnorderedIndices}{{}^{kj}{}_{jk}{}^{\mu}{}_{qa}}
    \end{array}
  }
\end{meta}
The interesting point introduced here is a special type of product indices. We
will discuss it in section. The general idea here is that from the mathematical
point of view the order of resulting product indices is undefined.
\subsubsection{List access operations}
\begin{meta}
  \alignArray{
    \begin{array}{rcl}
      \mobj{}{V_{\alpha\beta}M_\sigma
        Ric^{\kappa\delta}\,\delta^\alpha{}_{\kappa}}\mkey{.size()}&\alignSymbol&4\\[12pt]
      \mobj{}{H^{kj}M_jQ_k{}^{\mu}{}_{qa}}\mkey{.elements}&\alignSymbol&
      \mobj{List}{H^{kj},\,M_j,\,Q_k{}^{\mu}{}_{qa}}
    \end{array}
  }
\end{meta}
One of the important (and really hard) decisions we've made is to consider only
commutative theories. So, while processing products, the order of elements is
not taken into account! However, there are some tricks to work with
non-commutative theories.\newline Indeed, you can take an element by index:
\begin{meta}
  \alignArray{
    \begin{array}{rcl}
      \mobj{}{H^{kj}M_jQ_k{}^{\mu}{}_{qa}}\mkey{.get(2)}&\alignSymbol&
      \mobj{SimpleTensor}{Q_k{}^\mu{}_{qa}}
    \end{array}
  }
\end{meta}

\end{document} 

Answer 2

Another approach is to use a \makebox[<size>][l|r|c]{<text>} to set both the left hand and right hand side into a box of the exact size and place the \to arrow in between. Here I have assumed that you want the left hand side right aligned and the right hand side left aligned:

Notes:

• As requested in the comments, you can adjust \MetaHShift which controls how much the lines are shifted to the right. So, a negative value here will shift to the left. A zero value here will result in the \to arrow being in the center.

• The array should not be required. But empheq was now allowing me to put a new line in as in your second example, so used the array structure only for that. There probably is a way to do a new line without resorting to using array. Also note that the array options use @{} to eliminate the column spacing on the left and right.

• In my opinion usage of ensuremath within the \makebox improves readability, but a $...$ could have been used as well.

Code:

\documentclass[a4paper,titlepage,11pt,oneside]{book}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{amsthm}
\usepackage{latexsym}
\usepackage{array}
\usepackage{xargs}
\usepackage{xparse}
\usepackage{xcolor}
\usepackage{empheq}
\usepackage{courier}
\newcommand{\mkey}[1]{{\mbox{\small\ttfamily #1}}}
\newcommand{\mobj}[2]{\mbox{\ttfamily\small #1}\left [,#2,\right]}
\definecolor{boxcolor}{rgb}{.95, .95, 1}
\newsavebox{\mysaveboxM} % M for math
\newcommand*\MetaBox[2][Example]{%
 \sbox{\mysaveboxM}{#2}%
 \sbox{\mysaveboxM}{%
  \parbox[b][\ht\mysaveboxM][b]{%
  \wd\mysaveboxM}{#2}%
 }%
 \sbox{\mysaveboxM}{%
 \fcolorbox{black}{boxcolor}{%
  \makebox[\linewidth-4em]{\usebox{\mysaveboxM}}%
  }%
 }%
 \usebox{\mysaveboxM}%
}
\NewDocumentEnvironment{meta}{O{equation*}O{}}{%
    \setkeys{EmphEqEnv}{#1}%
    \setkeys{EmphEqOpt}{box=\MetaBox,#2}%
    \EmphEqMainEnv%
}{%
    \endEmphEqMainEnv%
}
%%
\newcommand{\MetaHShift}{-0.5em}
\newlength{\widthOfArrow}%
\newlength{\widthOfLeftText}%
\newlength{\widthOfRightText}%
\settowidth{\widthOfArrow}{${}\to{}$}%
\newcommand{\CentredArrow}[2]{%
    \setlength{\widthOfLeftText}{\dimexpr0.5\linewidth-0.5\widthOfArrow+\MetaHShift\relax}%
    \setlength{\widthOfRightText}{\dimexpr0.5\linewidth-0.5\widthOfArrow-\MetaHShift\relax}%
    \makebox[\widthOfLeftText][r]{\ensuremath{#1}}%
    \ensuremath{{}\to{}}%
    \makebox[\widthOfRightText][l]{\ensuremath{#1}}%
}%
\begin{document}
\subsection{Product}
....
\subsubsection{Indices}
\begin{meta}
\begin{array}{@{}l@{}}
\CentredArrow{\mobj{}{H^{kj}M_jQ_k{}^{\mu}{}{qa}}\mkey{.indices}}
{\mobj{UnorderedIndices}{{}^{kj}{}{jk}{}^{\mu}{}{qa}}}
\end{array}
\end{meta}
The interesting point introduced here is a special type of product
indices. We will discuss it in section. The general idea here is that

 from the mathematical point of view the order of resulting product indices

is undefined.
\subsubsection{List access operations}
\begin{meta}
\begin{array}{@{}l@{}}
\CentredArrow{\mobj{}{V{\alpha\beta}M_\sigma
Ric^{\kappa\delta},\delta^\alpha{}{\kappa}}\mkey{.size()}}{4}
\[12pt]
\CentredArrow{\mobj{}{H^{kj}M_jQ_k{}^{\mu}{}{qa}}\mkey{.elements}}{
\mobj{List}{H^{kj},,M_j,,Q_k{}^{\mu}{}{qa}}}
\end{array}
\end{meta}
One of the important (and really hard) decisions we've made is to
consider only commutative theories. So, while processing products, the order

of elements is not taken into account! However, there are some tricks to

work with non-commutative theories.\newline Indeed, you can take an element 
by index:
\begin{meta}
\begin{array}{@{}l@{}}
\CentredArrow{\mobj{}{H^{kj}M_jQ_k{}^{\mu}{}{qa}}\mkey{.get(2)}}{
\mobj{SimpleTensor}{Q_k{}^\mu{}_{qa}}}
\end{array}
\end{meta}
\end{document}