无左侧的多行等式链的对齐

Question

在排版包含长表达式的方程式链时，我喜欢将所有内容放在方程式符号的右侧，以避免框过满或使用multlined& 朋友。目前，我通过align将 & 符号放置为 的环境来执行此操作，但我刚刚注意到这会删除和 表达式=&之间的空格。=

IISC equation+split并alignat表现相同。我浏览了许多关于方程对齐的问题，但尚未找到这个确切的情况。我很高兴得到任何关于如何实现这一点的指示。

如果这不容易实现，我可能会考虑将第一行与=甚至稍微向左移动一点。

---

这是一个 MWe：

\documentclass{article}
\usepackage{amsmath}
\begin{document}
    This is what I'd like to avoid.
    \begin{align*}
        \text{First very long expression}
        &= \text{Second very long expression that might cause an overfull box}\\
        &= \text{Third very long expression}
    \end{align*}
    This is the alignment that I would like to achieve, but the \verb|\hphantom| seems like an ugly workaround.
    \begin{align*}
        &\hphantom{=.} \text{First very long expression}\\
        &= \text{Second very long expression that might cause an overfull box}\\
        &= \text{Third very long expression}
    \end{align*}
    Unfortunately \verb|align| removes the space between \verb|=| and expression when placing the \verb|&| right of the \verb|=|.
    \begin{align*}
        & \text{First very long expression}\\
        =& \text{Second very long expression that might cause an overfull box}\\
        =& \text{Third very long expression}
    \end{align*}
\end{document}

渲染结果为

Answer 1

={}& 将给出\mathrel= 就像$=$在 = 后不给出空格一样，但是$={}$将在 = 后给出空格。

\documentclass{article}
\usepackage{amsmath}
\begin{document}
Unfortunately \verb|align| removes the space between \verb|=| and expression when placing the \verb|&| right of the \verb|=|.
    \begin{align*}
        & \text{First very long expression}\\
        ={}& \text{Second very long expression that might cause an overfull box}\\
        ={}& \text{Third very long expression}
    \end{align*}
\end{document}

Answer 2

另一种放置方式是将第一个表达式放在对齐等号的稍微左边。

\documentclass{article}
\usepackage{amsmath,mathtools}

\usepackage[nopar]{lipsum}

\begin{document}

\lipsum[1][1-4]
\begin{align*}
\lefteqn{\text{First very long expression}}
\quad&\\
   &= \text{Second very long expression that might cause an overfull box}\\
   &= \text{Third very long expression}
\end{align*}
\lipsum[2][1-4]
\begin{align*}
\mathmakebox[1em][l]{\text{First very long expression}}&\\
   &= \text{Second very long expression that might cause an overfull box}\\
   &= \text{Third very long expression}
\end{align*}
\lipsum[3][1-4]

\end{document}

第二种方法不太隐秘，但本质上与第一种方法相同。