'For two instances of L{DelayedCall} C{a} and C{b}, C{a <= b} is true
if and only if C{a} is scheduled to run before C{b} or at the same
time as C{b}.'

def test_le(self):

    (zero, one) = (self.zero, self.one)
    self.assertTrue((zero <= one))
    self.assertFalse((one <= zero))
    self.assertTrue((zero <= zero))
    self.assertTrue((one <= one))
