@staticmethod
def _get_frame_limits(frm, flut):
    """
        Get the first and last index of a SpectrumStream frame

        In this way SpectrumStreamData[ix1:ix2] can be efficiently
        queried.
        """
    assert isinstance(frm, int), 'Must provide valid frame index'
    ix1 = flut[frm]
    try:
        ix2 = flut[frm + 1]
    except IndexError:
        ix2 = None
    return ix1, ix2
