# Aligning 'implies' sign but centre equations

I would like the implies sign to be aligned for a group of equations, I am able to do this using the alignat environment

\begin{alignat*}{1}
&\implies\quad x^2 + 2yx + 14y + y^2 = 5\\
&\implies\quad r^2 + z^2 = 10\\
\end{alignat*}


However, this results in all the equations being left-justified. I would like them to be centred while the implied sign remains aligned. Somewhat similar to doing

\begin{gather*}
\implies\quad x^2 + 2yx + 14y + y^2 = 5\\
\implies\quad r^2 + z^2 = 10\\
\end{gather*}


but with the implies sign as it was in the previously.

So, how do I align just 1 bit of a group of equations while everything else is centred?

As an alternative, here it is as a TABstack.

\documentclass{article}
\usepackage{amsmath}
\usepackage{tabstackengine}[2016-10-04]
\TABstackMath
\begin{document}
$\setstackgap{L}{1.3\baselineskip} \tabularCenterstack{rc}{ \implies& x^2 + 2yx + 14y + y^2 = 5\\ \implies& r^2 + z^2 = 10\\ \implies& r=5 }$
\end{document} More conventionally, the array environment could be used

\documentclass{article}
\usepackage{amsmath}
\begin{document}
$\renewcommand\arraystretch{1.3} \begin{array}{rc} \implies& x^2 + 2yx + 14y + y^2 = 5\\ \implies& r^2 + z^2 = 10\\ \implies& r=5 \end{array}$
\end{document}


You can use array with some tweaks for making it behave like an align* environment as far as vertical spacing and math style is concerned.

\documentclass{article}
\usepackage{amsmath}
\usepackage{array}

\newenvironment{impliesgathered}
{\renewcommand{\arraystretch}{1.2}
\begin{array}{r >{\displaystyle}c}}
{\end{array}}

\begin{document}

Here is the desired alignment
$\begin{impliesgathered} \Longrightarrow & x^2 + 2yx + 14y + y^2 = 5\\ \Longrightarrow & r^2 + z^2 = 10\\ \Longrightarrow & r=5 \end{impliesgathered}$
and here, for comparing the vertical spacing, the \texttt{align*}:
\begin{align*}
\Longrightarrow & x^2 + 2yx + 14y + y^2 = 5\\
\Longrightarrow & r^2 + z^2 = 10\\
\Longrightarrow & r=5
\end{align*}

\end{document} 