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