code: def get_under_hollow(self):
          
          C0 = self.B[-1:] * (3, 3, 1)
          ads_pos = C0.positions[4]
  
          C = self.get_subsurface_layer() * (3, 3, 1)
  
          ret = 'FCC'
          if np.any([np.linalg.norm(ads_pos[:2] - ele.position[:2]) < 0.5 *
                     cradii[ele.number] for ele in C]):
              ret = 'HCP'
  
          return ret
docs: Return HCP if an atom is present below the adsorbate in the 
          subsurface layer and FCC if not