Ignoring specific ampersand formatting on a specific line of an align block

Question

I want to typeset a number of equations with some alignment constraints, but I want to partially ignore them on certain lines. An example formatted text might look like the following when it's formatted

really really really long equation = a
      a = b                      b = c
      c = d                      d = e

where the equals signs in each column are aligned, but the top line ignores the first column of alignment. This is essentially the same question as Ignoring alignment for certain lines in an alignat environment, but that question wasn't answered satisfactorily.

The two naive ways both produce undesirable results.

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{align*}
  && \text{really really really long equation} &= a \\
  a &= b & b &= c
\end{align*}


or
\begin{align*}
  \text{really really really long equation} &= a \\
  a &= b & b &= c
\end{align*}

\end{document}

Answer 1

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\begin{align}
&& \llap{$\displaystyle \sum_0^7 x-(1+2+3+4+5+6+7)$} &= 0\\
      a& = b &                     b &= c\\
      c& = d  &                    d &= e
\end{align}

\end{document}

Answer 2

Use \span to merge cells. You need one \span per & you want to ignore. For example, your sample output could be produced like this:

\begin{align*}
  \text{really really really long equation} \span \span &= a \\ 
  a &= b & b &= c \\
  c &= d & d &= e
\end{align*}