code: def solvent_per_layer(self):
          
          if self._solvent_per_layer:
              return self._solvent_per_layer
  
          assert not (self.solvent_per_lipid is None and self.n_solvent is None)
          if self.solvent_per_lipid is not None:
              assert self.n_solvent is None
              self._solvent_per_layer = self.n_lipids_per_layer * self.solvent_per_lipid
          elif self.n_solvent is not None:
              assert self.solvent_per_lipid is None
              self._solvent_per_layer = self.n_solvent / 2
          return self._solvent_per_layer
docs: Determine the number of solvent molecules per single layer.