code: def get_element_masses(self, elements=None):
          
  
          if elements is None:
              elements = self.material.elements
          result = numpy.zeros(len(elements))
  
          for compound in self.material.compounds:
              result += self.get_compound_mass(compound) *\
                  numpy.array(stoich.element_mass_fractions(compound, elements))
          return result
docs: Determine the masses of elements in the package.
  
          :returns: Array of element masses. [kg]