def __get_key(peak1, peak2):
    """Generates a "key" from two peaks.

    Parameters
    ----------
    peak1, peak2 : List[Tuple[int, int]]
        The two peaks to generate a key from, where each peak is
        given by Tuple[<time>, <freq>]. Assumes `peak2` occurs concurrently or
        after `peak1`.

    Returns
    -------
    key : Tuple[int, int, int]
        `key` is given by Tuple[<freq_i>, <freq_n>, <dt>]
    """

    return peak1[1], peak2[1], peak2[0] - peak1[0]