def METHOD_NAME(self, i, j):
    """
    The product of two basis elements.
    The product of elements of degree ``i`` and ``j`` is an element
    of degree ``i+j``.
    INPUT:
    - ``i``, ``j`` -- non-negative integers
    OUTPUT:
    - a basis element indexed by ``i+j``
    TESTS::
        sage: H = GradedHopfAlgebrasWithBasis(QQ).Connected().example()
        sage: H.monomial(4) * H.monomial(5)
        P9
    """
    return self.monomial(i+j)