def modifyAsnMtyp(asn_mtyp):
    """Replace 'EXP' with 'PROD' in the ASN_MTYP keyword string.

    Parameters
    ----------
    asn_mtyp: str
        Value of ASN_MTYP keyword from an input file.

    Returns
    -------
    asn_mtyp: str
        asn_mtyp string, but with "EXP" replaced by "PROD".
    """
    if asn_mtyp.startswith('EXP-') or asn_mtyp.startswith('EXP_'):
        asn_mtyp = 'PROD' + asn_mtyp[3:]
    return asn_mtyp
