\documentclass[a4paper]{report}
\usepackage{amsmath,mathtools}
\makeatletter
\newcount\mw@shortcnt
\mw@shortcnt=0
\newcommand{\short}[1]{%
\global\advance\mw@shortcnt by +1%
\expandafter\def\csname temp\the\mw@shortcnt\endcsname{#1}
}
\newcount\mw@unshortcnt
\mw@unshortcnt=0
\newcommand{\unshort}{%
\global\advance\mw@unshortcnt by +1%
\csname temp\the\mw@unshortcnt\endcsname
}
\newcommand{\undershort}[1]{\underset{\unshort}{#1}}
\newcommand{\overshort}[1]{\overset{\unshort}{#1}}
\newcommand{\abbr}[2]{\expandafter\gdef\csname#1\endcsname{#2}}
\newcommand{\setshortcnt}[1]{\mw@shortcnt=#1}
\newcommand{\setunshortcnt}[1]{\mw@unshortcnt=#1}
\newcommand{\showshort}{\the\mw@shortcnt}
\newcommand{\showunshort}{\the\mw@unshortcnt}
\newcommand{\showboth}{\showshort\showunshort}
\makeatother
\newcommand{\ang}[1]{\left\langle #1\right\rangle}
\begin{document}
\showboth \\
\short{\mathclap{\substack{\text{posso portarmi da un membro all'altro la proiezione e $p_1$ idempotente.} \\ |}}}
\short{\mathclap{\substack{| \\ \text{commutano per ipotesi e $p_2$ porto a destra nel primo}}}}
\showboth \\
\begin{align*}
\showboth \\
\setunshortcnt{0}
\ang{x-p_1(p_2(x)\!),p_1(p_2(y)\!)}={}\text{\textit{\showboth}}&\text{\textit{\showboth}}\ang{x,p_1(p_2(y)\!)}-\ang{p_1p_2(x),p_1p_2(y)}\undershort{=}{} \\
{}={}&\ang{p_1(x),p_2(y)}-\ang{p_1(p_2(x)\!),p_2(y)}\undershort{=}\ang{p_2(p_1(x)\!),y}-\ang{p_1(x),p_2(y)}=0.
\end{align*}
\end{document}
The command \short is defined for when I have an \underset or \overset with \mathclap, \substack and \text combined in the under or over argument, as in the example, to keep the complications of that argument outside the equation and make the equation's code more readable. The command \unshort (and its abbreviation in combination with \underset and \overset) is the twin of \short to place the argument in place. They rely each on a counter, with the counters named \mw@shortcnt and \mw@unshortcnt, defining macros with a standard start followed by the counter's value (previously incremented globally to avoid conflict in sequences of \shorts) to expand to the argument that is to be later recovered, and recovering the argument placed in the \short when \mw@shortcnt was at a certain value (\mw@unshortcnt, previously globally incremented by 1). The example compiles to:
Both counters start at 0 as set. Then the shortcnt becomes 2 because of the two \shorts. And that's all right. Then I open the align* and the unshortcnt magically becomes 2! I set it back to 0 manually, and the following 20 shows the command is executed, but the & somehow undoes that! What is happening here? WHat does this align* environment do to my counters and why? After all it's prefixed with mw@, the counter name I mean, so how can amsmath know anything about it to tamper with it?
Update:
It looks like setting the unshortcnt right manually the first time is enough to solve this… why does the first & mess with the counter while the second one doesn't?
Edit: The idea of this is the following. If I write the above code with e.g.:
`\overset{\mathclap{\substack{\text{posso portarmi da un membro all'altro la proiezione e $p_1$ idempotente.} \\ |}{=}
It is next to unreadable. So I wanted to store the messy unreadable argument somewhere outside the equation, making things more readable in the equation. I started doing things like:
\def\temp{\mathclap{\substack{\text{posso portarmi da un membro all'altro la proiezione e $p_1$ idempotente.} \\ |}
To then recall the \temp macro inside the equation. Since it was a shortening and unshortening of the arguments, it seemed logical to have a macro to take care of the macro definition and recalling, and to create the name. Here came \short and \unshort. Since the most frequent use of this was in \underset, I combined \unshort with it, yielding \undershort. So the sole purpose of these macros was to make equation code more readable.
alignpasses over the material twice. – egreg Dec 10 '14 at 21:55unshortcntis modified. Also, ifalign*"passes over the material twice", then why is the second\global\advance\mw@unshortcnt+1not done twice? Setting it right once gets the second\undershortdealt with, yet if everything is executed twice in analign*, the second\undershortand the\advanceinside it should be done twice, resulting in a +2, but only a +1 happens. Maybe I misunderstood what you said @egreg. – MickG Dec 10 '14 at 22:01%in+1%– David Carlisle Dec 10 '14 at 22:08%it increments the counter by 1, with the%it may or may not increment by 1 depending on the expansion of\csname temp\the\mw@unshortcnt\endcsnameif its expansion began with2then the counter would increment by 12 – David Carlisle Dec 10 '14 at 22:21\csname … \endcsnameis done before the advance, another thing I didn't really know. I'd have expected the advance to happen either before the expansion line was even looked at or maybe after the expansion of\csnameto form the token but before the expansion of the complex tokentemp<value of mw@unshortcnt>. – MickG Dec 10 '14 at 22:29\advanceneeds a%forces the\csnameto be expanded before the\advancewithout the%it expands after the\advance.