Evaluate key-subkey non-existence.

    Checks that the key-subkey combo does not exists in the
    configuration options.

def _entry_must_not_exist(df, k1, k2):
    """Evaluate key-subkey non-existence.

    Checks that the key-subkey combo does not exists in the
    configuration options.
    """
    count = df[(df['k1'] == k1) &
               (df['k2'] == k2)].shape[0]
    if count > 0:
        raise AlreadyRegisteredError(
            "Option {0}.{1} already registered".format(k1, k2))