VISITORS They landed at last, touching down in Central Park in their gorgeously decorated ships like maple leaves falling precisely; and were received with respect and ticker-tape. Not repulsive after all, but tall and long-tailed, furry, they walked with slinky grace like black teenagers. Their gift is all the wisdom of the civilised Galaxy "The cure for cancer? This is simple. At the first full moon after the Winter Solstice, decapitate a black cockerel; drain the blood into a silver cup . . . " et cetera. Are they serious?