'Computes the ``m``-th order derivative of ``f`` in ``x_j``.'

def diff(f, m=1, j=0):

    if (not isinstance(m, int)):
        raise TypeError(('``int``    expected,    got    %s' % type(m)))
    if (not isinstance(j, int)):
        raise TypeError(('``int``    expected,    got    %s' % type(j)))
    return f.per(dmp_diff_in(f.rep, m, j, f.lev, f.dom))
