def _doublet(plist, J):
    """
    Applies a *J* coupling to each signal in a list of (frequency, intensity)
    signals, creating two half-intensity signals at +/- *J*/2.

    Parameters
    ---------
    plist : [(float, float)...]
        a list of (frequency{Hz}, intensity) tuples.
    J : float
        The coupling constant in Hz.

    Returns
    -------
    [(float, float)...]
        a list of (frequency, intensity) tuples.
    """
    res = []
    for v, i in plist:
        res.append((v - J / 2, i / 2))
        res.append((v + J / 2, i / 2))
    return res
