private Optional<DoubleBinaryOperator> asFunctionExpression(DoubleBinaryOperator operator) {
        return Optional.of(new DoubleBinaryOperator() {
            @Override
            public double applyAsDouble(double left, double right) {
                return operator.applyAsDouble(left, right);
            }
            @Override
            public String toString() {
                return LambdaFunctionNode.this.toString();
            }
        });
    }
Returns this as a double binary operator

@throws IllegalStateException if this has more than two arguments
searchlib/src/main/java/com/yahoo/searchlib/rankingexpression/rule/LambdaFunctionNode.java