code: def method(self, symbol):
          '''
          Symbol decorator.
          '''
          assert issubclass(symbol, SymbolBase)
          def wrapped(fn):
              setattr(symbol, fn.__name__, fn)
          return wrappef method(self, symbol):
          '''
          Symbol decorator.
          '''
          assert issubclass(symbol, SymbolBase)
          def wrapped(fn):
              setattr(symbol, fn.__name__, fn)
          return wrapped
docs: Symbol decorator.