code: def global_symbols_in_children(self):
          
  
          result = set()
          for child in self.children:
              result |= (
                  child.global_symbols |
                  child.global_symbols_in_children)
          return result
docs: This is based on all children referenced symbols that have not
          been declared.
  
          The intended use case is to ban the symbols from being used as
          remapped symbol values.