[XeTeX] Eq multi-labeling problem

[snvv]

Hello,
I when i set a multi-labeling equation the results is like (1a ) (1 b) etc 
(see attached file).

Is it possible to make that numbering (1a) (1b) or (1-1) (1-2) etc ?


Thank you
snvv

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
I use the following code
%!TEX TS-program = xelatex -output-driver="xdvipdfmx -q -E"
%!TEX encoding = UTF-8 Unicode
\documentclass[a4papper,oneside,12pt]{book}
\usepackage{amssymb} 
\usepackage{amsmath} 
\usepackage{fontspec,fontenc,xunicode,xltxtra,xgreek} % 


and 

\begin{subequations}\label{grp}
\begin{align}
a&=b+c\label{second}\\
d&=e+f+g\label{third}\\
h&=i+j\label{fourth}
\end{align}
\end{subequations}