@Override
    public BucketSet getBucketSet(BucketIdFactory factory) {
        if (operator.equals("==") || operator.equals("=")) {
            if (lhs instanceof IdNode && rhs instanceof LiteralNode) {
                return compare(factory, (IdNode)lhs, (LiteralNode)rhs, operator);
            } else if (rhs instanceof IdNode && lhs instanceof LiteralNode) {
                return compare(factory, (IdNode)rhs, (LiteralNode)lhs, operator);
            }
        }
        return null;
    }
Returns the right hand side of this comparison.

@return The right hand side expression.
document/src/main/java/com/yahoo/document/select/rule/ComparisonNode.java