code: def composite_join(separator, items):
          
          output = Composite()
          first_item = True
          for item in items:
              # skip empty items
              if not item:
                  continue
              # skip separator on first item
              if first_item:
                  first_item = False
              else:
                  output.append(separator)
              output.append(item)
          return output
docs: Join a list of items with a separator.
          This is used in joining strings, responses and Composites.
          The output will be a Composite.