code: def number_of_each_lipid_per_layer(self):
          
          if self._number_of_each_lipid_per_layer:
              return self._number_of_each_lipid_per_layer
  
          for lipid in self.lipids[:-1]:
              self._number_of_each_lipid_per_layer.append(int(round(lipid[1] * self.n_lipids_per_layer)))
  
          # TODO: give warning if frac * n different than actual
          # Rounding errors may make this off by 1, so just do total - whats_been_added.
          self._number_of_each_lipid_per_layer.append(self.n_lipids_per_layer - sum(self._number_of_each_lipid_per_layer))
          assert len(self._number_of_each_lipid_per_layer) == len(self.lipids)
          return self._number_of_each_lipid_per_layer
docs: The number of each lipid per layer.