code: def set_blend_func(self, srgb='one', drgb='zero',
                         salpha=None, dalpha=None):
          
          salpha = srgb if salpha is None else salpha
          dalpha = drgb if dalpha is None else dalpha
          self.glir.command('FUNC', 'glBlendFuncSeparate', 
                            srgb, drgb, salpha, dalpha)
docs: Specify pixel arithmetic for RGB and alpha
      
          Parameters
          ----------
          srgb : str
              Source RGB factor.
          drgb : str
              Destination RGB factor.
          salpha : str | None
              Source alpha factor. If None, ``srgb`` is used.
          dalpha : str
              Destination alpha factor. If None, ``drgb`` is used.