def METHOD_NAME(self):
    """
    Return the symmetric bilinear form preserved by the orthogonal
    group.
    OUTPUT:
    A matrix `M` such that, for every group element `g`, the
    identity `g m g^T = m` holds. In characteristic different from
    two, this uniquely determines the orthogonal group.
    EXAMPLES::
        sage: G = GO(4, GF(7), -1)
        sage: G.invariant_bilinear_form()
        [0 1 0 0]
        [1 0 0 0]
        [0 0 2 0]
        [0 0 0 2]
        sage: G = GO(4, GF(7), +1)
        sage: G.invariant_bilinear_form()
        [0 1 0 0]
        [1 0 0 0]
        [0 0 6 0]
        [0 0 0 2]
        sage: G = SO(4, GF(7), -1)
        sage: G.invariant_bilinear_form()
        [0 1 0 0]
        [1 0 0 0]
        [0 0 2 0]
        [0 0 0 2]
    TESTS::
        sage: G.invariant_form()
        [0 1 0 0]
        [1 0 0 0]
        [0 0 2 0]
        [0 0 0 2]
    """
    m = self.gap().InvariantBilinearForm()['matrix'].matrix()
    m.set_immutable()
    return m