Monoclonal antibodies VTB‐E3, IB5 and HB9 to the leucocyte/epithelial antigen CD24 resemble BA‐1 in recognizing sialic acid‐dependent epitope(s). Evidence that VIB‐E3 recognizes NeuAcα2‐6GalNAc and NeuAcα2‐6Gal sequences