Sub-numbering equations within array

Question

I'm using the array environment to give multiple definitions of some value, let's call it x. I can easily get a reference to all 3 definitions together, but I want each one to have its own reference. For now it looks like this :

    / a + b
x = | c + d    (1)
    \ e + f

And I want it to look like this :

    / a + b    (1.a)
x = | c + d    (1.b)
    \ e + f    (1.c)

I've tried combining all sorts of other packages with array and they just won't play nice. Any ideas? Here's the code I use to get the first result :

\begin{equation}
x = \left\{ \begin{array}{rl}
  & a + b \\
  & c + d \\
  & e + f
\end{array} \right.
\label{eq:PSF_mag}
\end{equation}

Answer 1

The empheq package was made for this:

\documentclass[a4paper]{memoir}
\usepackage{empheq} % autoload amsmath
\begin{document}

\begin{subequations}
\label{eq:tot}
\begin{empheq}[left={x=}\empheqlbrace]{align}
  & a + b \\
  & c + d \label{eq:b}\\
  & e + f
\end{empheq}
\end{subequations}

Total group: \eqref{eq:tot}, just one: \eqref{eq:b}

\end{document}

Answer 2

The subnumcases environment from cases helps for this layout:

\documentclass{article}
\usepackage{cases}% http://ctan.org/pkg/cases

\begin{document}

\begin{subnumcases}{x=}
  a + b \\
  c + d \\
  e + f
\end{subnumcases}

\end{document}

Subequation numbering can be modified using (say)

\renewcommand{\thesubequation}{\themainequation.\alph{equation}}