code: def get_reduced_symbols(symbols):
      
  
      reduced_symbols = []
  
      for ss in symbols:
          if not (ss in reduced_symbols):
              reduced_symbols.append(ss)
  
      return reduced_symbols
docs: Reduces expanded list of symbols.
  
      Args:
          symbols: list containing any chemical symbols as often as
          the atom appears in the structure
  
      Returns:
          reduced_symbols: any symbols appears only once