AC_DEFUN: default values of optional arguments

[Sam Steingold]

How do I give an optional argument its default value?
e.g.,
AC_DEFUN([FOO],[...])
FOO() should be equivalent to FOO([true])
do I write something like
AC_DEFUN([FOO],[
opt=$2;
test -z "$opt" && opt=true
])
??

thanks.
Sam.