def _check_unique_id(base, tag, id):
    """Find all child elements with a given group and check if the id is already used.

    Args:
        base: The base node element. (lxml node)
        tag: Child elements group tag used to select the elements. (string)
        id: The unique id to search for. (string)

    Returns:
        False if the id is already used, True if not.
    """
    for node in base:
        if node.tag.replace('{http://www.rdml.org}', '') == tag:
            if node.get('id') == id:
                return False
    return True
