Add an element to the <keyword>'s list of refinements

def add_refinement_element(keyword, element, merge = False, v_fun=None):
    """Add an element to the <keyword>'s list of refinements"""
    for (key, valid_keywords, m, v_fun) in _refinements:
        if key == keyword:
            valid_keywords.append(element)
            return
    _refinements.append((keyword, [element], merge, v_fun))