def _get_first_child_text(base, tag):
    """Get a child element of the base node with a given tag.

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

    Returns:
        The text of first child node element found or an empty string.
    """
    for node in base:
        if node.tag.replace('{http://www.rdml.org}', '') == tag:
            return node.text
    return ''
