Structure elucidation of blood group B-like and I-active ceramide eicosa- and pentacosa-saccharides from rabbit erythrocyte membranes by combined gas chromatography-mass spectrometry; electron-impact and fast-atom-bombardment mass spectrometry; and two-dimensional correlated, relayed-coherence transfer, and nuclear overhauser effect 500-MHz 1H-N.m.r. spectroscopy