mathtools: tagging with words and a number in align

mathtools: tagging with words and a number in align

How does one tag an equation with both words and numbers?
\documentclass{article}
\usepackage{mathtools}
\newenvironment{Dequation}
{%
\def\tagform@##1{%
\maketag@@@{\makebox[0pt][r]{(\ignorespaces##1\unskip\@@italiccorr)}}}%
\ignorespaces
}
{%
\def\tagform@##1{\maketag@@@{(\ignorespaces##1\unskip\@@italiccorr)}}%
\ignorespacesafterend
}
\makeatother
\begin{document}
\begin{Dequation}
\begin{align*}
3 &= 3\\
4 &= 4\\
6 &= 6\tag{since $x_0 = 0$}\label{six}
\end{align*}
\end{Dequation}
\end{document}
So I want to reference my equation, but I want the reference to be
equation (1). Therefore, it would seem strange if the equation wasn't
numbered if someone was looking for that reference.