Interrupt environment with command

Question

At the moment I'm trying to figure out how I could interrupt an environment. What I did so far...

\documentclass{article}
\usepackage{amsmath}

\newenvironment{envnew}
{
    \begingroup
    \csname align*\endcsname
}{
    \csname endalign*\endcsname
    \endgroup
}
\newcommand{\last}[1]{%
    \end{envnew}\begin{envnew}#1%
}

\begin{document}
\begin{envnew}
a &= b + c\\
x &= y + z\\
a &= b + c\\
x &= y + z
\last{\text{content}}
\end{envnew}
\end{document}

I think most of it should be self-explaining, I'm just trying to append a second environment at the end - background should be unimportant. Most of the given code works, but it breaks with the error...

! Missing $ inserted.
<inserted text>
                $
l.24 \end{envnew}

...any idea what I'm doing wrong?

Answer 1

amsmath has more than one multi-line math environment, and it's possible to use more than one for a single display to obtain the desired results. sometimes it's also necessary to be devious if material above and below the line of "text" has expressions of different widths on the left and right of the alignment point. here's a contrived example.

\documentclass{article}
\usepackage{amsmath}

\begin{document}
\begin{gather*}
\begin{aligned}
\phantom{a + b}\llap{$a$} &= b + c\\
x &= y + z\\
a &= b + c\\
x &= y + z
\end{aligned}\\
\text{content}\\
\begin{aligned}
a + b &= c\\
x &= y + z
\end{aligned}
\end{gather*}
\end{document}

the \llapis used with a phantom of the longest segment in the aligned portions of the composite display to ensure mutual alignment of the separated components. (if a longer segment were present on the right side, an \rlap would be used, placed just after the sign of relation.)