code: def elements(compounds):
      
  
      elementlist = [parse_compound(compound).count().keys()
                     for compound in compounds]
      return set().union(*elementlist)
docs: Determine the set of elements present in a list of chemical compounds.
  
      The list of elements is sorted alphabetically.
  
      :param compounds: List of compound formulas and phases, e.g.
        ['Fe2O3[S1]', 'Al2O3[S1]'].
  
      :returns: List of elements.