code: def add(self, child, min_occurs=1):
          
          if not min_occurs in (0, 1):
              raise RuntimeError("Unexpected min bound for node schema")
          self.children[child.tag] = child
          self.children_min_occurs[child.tag] = min_occurs
          return child
docs: Add a child node.
  
          @param child: The schema for the child node.
          @param min_occurs: The minimum number of times the child node must
              occur, if C{None} is given the default is 1.