docs: def merge(cls, first, second):
          """
          Return an AttributeList that is the result of merging first with second.
          """
          merged = AttributeList([], None)
  
          assert (isinstance(first, AttributeList))
          assert (isinstance(second, AttributeList))
          merged._contents = first._contents[:]
          merged._contents += second._contents[:]
  
          return merged