code: def set_stencil_func(self, func='always', ref=0, mask=8, 
                           face='front_and_back'):
          
          self.glir.command('FUNC', 'glStencilFuncSeparate', 
                            face, func, int(ref), int(mask))
docs: Set front or back function and reference value
      
          Parameters
          ----------
          func : str
              See set_stencil_func.
          ref : int
              Reference value for the stencil test.
          mask : int
              Mask that is ANDed with ref and stored stencil value.
          face : str
              Can be 'front', 'back', or 'front_and_back'.