code: def add_to(self, other):
          
  
          # Add another package.
          if type(other) is MaterialPackage:
  
              # Packages of the same material.
              if self.material == other.material:
                  self.compound_masses += other.compound_masses
  
              # Packages of different materials.
              else:
                  for compound in other.material.compounds:
                      if compound not in self.material.compounds:
                          raise Exception("Packages of '" + other.material.name +
                                          "' cannot be added to packages of '" +
                                          self.material.name +
                                          "'. The compound '" + compound +
                                          "' was not found in '" +
                                          self.material.name + "'.")
                      self.add_to((compound, other.get_compound_mass(compound)))
  
          # Add the specified mass of the specified compound.
          elif self._is_compound_mass_tuple(other):
              # Added material variables.
              compound = other[0]
              compound_index = self.material.get_compound_index(compound)
              mass = other[1]
  
              # Create the result package.
              self.compound_masses[compound_index] += mass
  
          # If not one of the above, it must be an invalid argument.
          else:
              raise TypeError('Invalid addition argument.')
docs: Add another chem material package to this material package.
  
          :param other: The other material package.