code: def molecules2symbols(molecules, add_hydrogen=True):
      
      symbols = sorted(
          list(set(
              ase.symbols.string2symbols(''.join(
                  map(
                      lambda _x:
                      ''.join(ase.symbols.string2symbols(_x)), molecules)
              ))
          )),
          key=lambda _y: ase.data.atomic_numbers[_y])
  
      if add_hydrogen and 'H' not in symbols:
          symbols.insert(0, 'H')
  
      return symbols
docs: Take a list of molecules and return just a list of atomic
      symbols, possibly adding hydrogen