# How to flush left single alignat environments

Im looking for a way to "flush left" single alignat-environments, like in the flalign-environment. This, however doesn't suit my needs, as I want to align equation marks of different lines and have other, left-aligned lines, which are possibly longer and left-aligned using \mathrlap. Using flalign naturally shifts the equation marks to the right. (fleqn is no option, as most equations in the document should be centered.)

\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}


• note the fl in flalign is not for "flush left" but "full length", we have answers somewhere for switching between fleqn and centred settings mid-document, I'll search.. – David Carlisle Jul 17 '17 at 8:23
• @DavidCarlisle thanks for the hint "full length". Still the problem remains the same. I found a few things about that but none fit my needs :( – Ktree Jul 17 '17 at 11:42
• it's very hard to suggest any answer as it isn't clear why you want to use align... there is no alignment here all there terms are in mathrlap so there alignment even possible as the size of the terms is hidden, it would seem gather or multline would be far more suitable – David Carlisle Jul 18 '17 at 20:09
• The = shall be aligned. There can more lines and using phantom is not as easy as it is in the MWE – Ktree Jul 19 '17 at 4:06