code: func (s *Struct) addSymbol(sym Symbol) {
  	if s.frozen {
  		panic("frozen")
  	}
  	s.symbols = append(s.symbols, sym)
  }
docs: // addSymbol appends a symbol to the symbols list in the struct.
  //
  // Note that starlark forbids reassigning variables in the module scope, so we
  // don't check that 'sym' wasn't added before.
  //
  // Panics if the struct is frozen.