Numbering just one equation from aligned set of equations when using the witharrows package

Question

For some mathematical derivations, I am using the witharrows package.

How can I typeset the equation number for the last equation and assign a label to it for later cross-references in text?

The following is a MWE

\documentclass{article}
\usepackage{witharrows}
\begin{document}

$\begin{WithArrows}[displaystyle]
    {(a + b)}^2 &= {(a + b)}{(a + b)} \Arrow{FOIL operations} \\
    {} & = a\cdot a + a \cdot b + b \cdot a + b \cdot b \Arrow{evaluate each sub-expression}\\
    {} & = a^2 + ab + ba + b^2 \Arrow{scalar multiplication is commutative}\\
    {(a + b)}^2 & = a^2 + 2ab + b^2 % <---- typeset this equation and assign label/tag, e.g. eq:aplusbsquared
\end{WithArrows}$

As seen in eq~\ref{eq:aplusbsquared}, blah blah ....
\end{document}

I would like to assign an equation number to the last equation and cross-reference it later in text.

I tried all standard methods. \label, \tag etc. do not work since this package is based on aligned environment. Any help will be much appreciated.

Answer 1

The version 1.7 of witharrows (2018, Jul 18) provides new environments {DispWithArrows} and {DispWithArrows*} which give a way to control numbers, tags and labels in a way similar of the environments {align} and {align*} of amsmath.

\begin{DispWithArrows}
A & = B \Arrow{text} \notag \\
  & = C \notag \\
  & = D \notag \\
  & = E \label{my-equation}
\end{DispWithArrows}

Reference to the equation \ref{my-equation}.

Answer 2

I would use the option b of the environment WithArrows:

\begin{equation}
   \begin{WithArrows}[b,displaystyle]
   line 1 \\
   line 2 \\
   ...
   last line % without \\
   \end{WithArrows}
   \label{...}
\end{equation}