@Override
    public CompositeNode setChildren(List<ExpressionNode> children) {
        if ( children.size() != 1)
            throw new IllegalArgumentException("A lambda function must have a single child expression");
        return new GeneratorLambdaFunctionNode(type, children.get(0));
    }
Returns this as an operator which converts a list of integers into a double.
searchlib/src/main/java/com/yahoo/searchlib/rankingexpression/rule/GeneratorLambdaFunctionNode.java