code: def substrate_a(self, **kwargs):
          '''
          Returns the substrate's lattice parameter.
          '''
          if self.substrate is not None:
              return self.substrate.a(**kwargs)
          else:
              return (self.unstrained.a(**kwargs) /
                      (1. - self.strain_in_plane(**kwargs))f substrate_a(self, **kwargs):
          '''
          Returns the substrate's lattice parameter.
          '''
          if self.substrate is not None:
              return self.substrate.a(**kwargs)
          else:
              return (self.unstrained.a(**kwargs) /
                      (1. - self.strain_in_plane(**kwargs)))
docs: Returns the substrate's lattice parameter.