code: def add(self, schema):
          
          key = schema.tns[1]
          existing = self.namespaces.get(key)
          if existing is None:
              self.children.append(schema)
              self.namespaces[key] = schema
          else:
              existing.root.children += schema.root.children
              existing.root.nsprefixes.update(schema.root.nsprefixes)
docs: Add a schema node to the collection.  Schema(s) within the same target
          namespace are consolidated.
          @param schema: A schema object.
          @type schema: (L{Schema})