The method was implementated as:

def check_representation(self):
    """
    Checks that the internal representation is correct.

    :raise: AssertionError if an error is found.
    """
    for attribute in (self.max, self.head, self.size):
        assert attribute >= 0, "{0} shouldn't be negative. ({1})".format(attribute, str(self))
    assert ((self.head + self.size) % self.max == self.tail), str(self)
    return

Notes

This is for the fixed-size queue used in udacity's Software Testing class
Since the second assertion checks an equation where the tail can never be negative if all the left-hand terms are positive, the tail isn't checked to see if it's positive