code: def add_assay(self, name, assay):
          
  
          if not type(assay) is list:
              raise Exception('Invalid assay. It must be a list.')
  
          elif not len(assay) == self.compound_count:
              raise Exception('Invalid assay: It must have the same number of '
                              'elements as the material has compounds.')
  
          elif name in self.assays:
              raise Exception('Invalid assay: An assay with that name already '
                              'exists.')
  
          self.assays[name] = assay
docs: Add an assay to the material.
  
          :param name: The name of the new assay.
          :param assay: A list containing the compound mass fractions for
            the assay. The sequence of the assay's elements must correspond to
            the sequence of the material's compounds.