如何刷新左单对齐环境

Question

我正在寻找一种方法来“向左对齐”单个alignat-环境，就像在flalign-环境中一样。然而，这并不符合我的需要，因为我想对齐不同行的方程式标记，并有其他左对齐的行，这些行可能更长，并使用 左对齐\mathrlap。使用flalign自然会将方程式标记向右移动。（fleqn没有选择，因为文档中的大多数方程式都应该居中。）

\documentclass[varwidth=10cm, border=2pt
,convert={ghostscript, density={500}}
]{standalone}
\usepackage{amsmath,amssymb,mathtools}

\newcommand{\mathphantlong}{
    a x^2 + bx^2 + cx^4 + dx^5 + ex^6 + fx^7 + \ldots
}
\newcommand{\mathphantshorta}{
    bu + cu^2
}
\newcommand{\mathphantshortb}{
    d y + e\sqrt{y}
}
\begin{document}
It should look like this:
    \begin{subequations}
        \begin{alignat}{2}
            & \mathrlap{\underline{i = j:}}\nonumber\\[0.1cm]
            & \mathrlap{\mathphantlong} \\
            &&[\mathphantshorta &= \mathphantshortb]_{i \leq m}\\
            &\mathrlap{[ \mathphantlong}\nonumber\\
            &&& = \mathphantshortb]_{i > m}
        \end{alignat}
    \end{subequations}
But left aligned like in:
    \begin{subequations}
        \begin{flalign}
            & \mathrlap{\underline{i = j:}}\nonumber\\[0.1cm]
            & \mathrlap{\mathphantlong} \\
            &&[\mathphantshorta &= \mathphantshortb]_{i \leq m}\\
            &\mathrlap{[ \mathphantlong}\nonumber\\
            &&& = \mathphantshortb]_{i > m}
        \end{flalign}
    \end{subequations}
\end{document}