Is it possible to break up an align environment and keep the tab positions?

Question

I'd like to take notes for a math class such that I can typeset a series of equations with the \align environment and make some text notes in between some of the lines. As far as I can tell, though... by using \end{align*}, typing some text notes about the process, and then starting a new \begin{align*}, I won't be able to align the = symbols in the above environment with & anymore.

The best I could come up with is to use \text{} in the middle of the equations, but that doesn't place the text along the left margin of the document.

\documentclass{article}

\usepackage[fleqn]{mathtools}

\begin{document}
  This is a series of equations:
  \begin{align*}
    x^{2}+y^{2}&=0\\
    y&=mx+b\\
    \text{This is a note about the above lines}\\
    &\text{This is another note}\\
    x^{2}&=1
  \end{align*}
\end{document}

Answer 1

amsmath provides \intertext, which interrupts your align, and lets you add some text. Sometimes, however, the spacing around the \intertext is too large, so the mathtools package provides \shortintertext. Compare:

\usepackage{mathtools}

\begin{align}
a & = b \\
\intertext{Some note.}
c & = d \\
\shortintertext{Another note.}
e & = f
\end{align}