code: private SqlNode navigationInDefine(SqlNode node, String alpha) {
  		Set<String> prefix = node.accept(new PatternValidator(false));
  		Util.discard(prefix);
  		node = new NavigationExpander().go(node);
  		node = new NavigationReplacer(alpha).go(node);
  		return node;
  	}
docs: Checks that all pattern variables within a function are the same,
  and canonizes expressions such as {@code PREV(B.price)} to
  {@code LAST(B.price, 0)}.