The center of a group $G$ is defined to be the set of all elements $x$ in $G$ such that $xy = yx$ for all $y$ in $G$.

Consider the group whose group table is given as follows ( $e$ is the identity element ):

	$e$	$x1$	$x2$	$x3$	$x4$	$x5$	$x6$	$x7$
$e$	$e$	$x1$	$x2$	$x3$	$x4$	$x5$	$x6$	$x7$
$x1$	$x1$	$x2$	$x3$	$e$	$x5$	$x6$	$x7$	$x4$
$x2$	$x2$	$x3$	$e$	$x1$	$x6$	$x7$	$x4$	$x5$
$x3$	$x3$	$e$	$x1$	$x2$	$x7$	$x4$	$x5$	$x6$
$x4$	$x4$	$x7$	$x6$	$x5$	$x2$	$x1$	$e$	$x3$
$x5$	$x5$	$x4$	$x7$	$x6$	$x3$	$x2$	$x1$	$e$
$x6$	$x6$	$x5$	$x4$	$x7$	$e$	$x3$	$x2$	$x1$
$x7$	$x7$	$x6$	$x5$	$x4$	$x1$	$e$	$x3$	$x2$

Using this group table, determine the elements that lie in the center of $G$ and enter them as a comma-separated list.