private void extractBindTargets(ExpressionNode node, Set<String> bindTargets) {
            if (node instanceof ReferenceNode) {
                if (((ReferenceNode)node).getArguments().expressions().size() > 0)
                    throw new UnsupportedOperationException("Can not bind " + node +
                                                            ": Array lookup is not supported with features having arguments)");
                bindTargets.add(node.toString());
            }
            else if (node instanceof CompositeNode cNode) {
                for (ExpressionNode child : cNode.children())
                    extractBindTargets(child, bindTargets);
            }
        }
Value to return if value is missing.
searchlib/src/main/java/com/yahoo/searchlib/rankingexpression/evaluation/AbstractArrayContext.java