def _swift_bool(bstring):
    """
    Convert a SWIFT-VOevent style boolean string ('true'/'false') to a bool.
    """
    if bstring == 'true':
        return True
    elif bstring == 'false':
        return False
    else:
        raise ValueError("This string does not appear to be a SWIFT VOEvent "
                          "boolean: %s" % bstring)