How to use the parsel.xpathfuncs.set_xpathfunc function in parsel

myfunc.call_count = 0

        body = u"""
        <p class="foo">First</p>
        """
        sel = Selector(text=body)
        self.assertRaisesRegexp(
            ValueError, 'Unregistered function in myfunc',
            sel.xpath, 'myfunc()')

        set_xpathfunc('myfunc', myfunc)
        sel.xpath('myfunc()')
        self.assertEqual(myfunc.call_count, 1)

        set_xpathfunc('myfunc', None)
        self.assertRaisesRegexp(
            ValueError, 'Unregistered function in myfunc',
            sel.xpath, 'myfunc()')

def test_set_xpathfunc(self):

        def myfunc(ctx):
            myfunc.call_count += 1

        myfunc.call_count = 0

        body = u"""
        <p class="foo">First</p>
        """
        sel = Selector(text=body)
        self.assertRaisesRegexp(
            ValueError, 'Unregistered function in myfunc',
            sel.xpath, 'myfunc()')

        set_xpathfunc('myfunc', myfunc)
        sel.xpath('myfunc()')
        self.assertEqual(myfunc.call_count, 1)

        set_xpathfunc('myfunc', None)
        self.assertRaisesRegexp(
            ValueError, 'Unregistered function in myfunc',
            sel.xpath, 'myfunc()')