def is_peer_reviewed(df):
    """Adds column for whether the paper is peer reviewed.
    
    Parameters
    ----------
    df: DataFrame
        input dataframe
    
    Returns
    -------
    Series
        series of whether each paper is peer reviewed

    """
    from_pr_source = df["source_x"].str.contains("PMC") | df["source_x"].str.contains("Medline") | df["source_x"].str.contains("Elsevier")
    from_nonpr_source = df["source_x"].str.contains("ArXiv") | df["source_x"].str.contains("BioRxiv") | df["source_x"].str.contains("MedRxiv")
    return from_pr_source & ~from_nonpr_source