code: def append(self, child):
          
          if not (isinstance(child, CP2KSection) or isinstance(child, CP2KKeyword)):
              raise TypeError("The child must be a CP2KSection or a CP2KKeyword, got: %s." % child)
          l = self.__index.setdefault(child.name, [])
          l.append(child)
          self.__order.append(child)
docs: Add a child section or keyword