def insert_sil(phones):
    """
    Insert silences.

    Parameters
    ----------
    phones : list
        A list of phone sequence

    Returns
    -------
    out : list
        A list of new labels.

    """
    
    out = []
    for i,(s,e,p) in enumerate(phones):
        
        if out:
            if out[-1][1]!=s:
                out.append((out[-1][1],s,'[SIL]'))
        out.append((s,e,p))
    return out