def METHOD_NAME(self):
    r"""
    The degree of ``self``.
    OUTPUT:
    The degree of this homomorphism. Raise an error if this is
    the trivial homomorphism.
    EXAMPLES::
        sage: from sage.modules.fp_graded.free_module import FreeGradedModule
        sage: A = SteenrodAlgebra(2)
        sage: homspace = Hom(FreeGradedModule(A, (0,1)), FreeGradedModule(A, (0,)))
        sage: N = homspace.codomain()
        sage: values = [Sq(5)*N.generator(0), Sq(3,1)*N.generator(0)]
        sage: f = homspace(values)
        sage: f.degree()
        5
    The zero homomorphism has no degree::
        sage: homspace.zero().degree()
        Traceback (most recent call last):
        ...
        ValueError: the zero morphism does not have a well-defined degree
    """
    if self._degree is None:
        # The zero morphism has no degree.
        raise ValueError("the zero morphism does not have a well-defined degree")
    return self._degree