def extract_repeated_attr(param, field):
    """
    Extract a particular field defined as a RepeatedContainer

    Parameters
    ----------
    param : parameter object
       Various parameter objects defined by Google messages.
    field : string
       Parameter field.

    Notes
    -----
    Only the first value is returned.

    Returns
    -------
    string or None
        Field value or None if parameter or field doesn't exist.

    """
    tmpval = getattr(param, field, None)
    if tmpval is not None:
        if isinstance(tmpval, (float, int, bool)):
            y = tmpval
        elif len(tmpval) > 0:
            y = tmpval[0]
        else:
            y = None
        return y
    else:
        return None
