code: def get_element_mfrs(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_mfr(compound) *\
                  stoich.element_mass_fractions(compound, elements)
          return result
docs: Determine the mass flow rates of elements in the stream.
  
          :returns: Array of element mass flow rates. [kg/h]