code: def _check_symbols(self, symbols):
          
          if len(symbols) != self.size:
              raise TypeError("The number of symbols in the graph does not "
                  "match the length of the atomic numbers array.")
          for symbol in symbols:
              if not isinstance(symbol, str):
                  raise TypeError("All symbols must be strings.")
docs: the size must be the same as the length of the array numbers and all elements must be strings