public DoubleUnaryOperator asDoubleUnaryOperator() {
        if (arguments.size() > 1)
            throw new IllegalStateException("Cannot apply " + this + " as a DoubleUnaryOperator: " +
                                            "Must have at most one argument " + " but has " + arguments);
        return new DoubleUnaryLambda();
    }
Returns this as a double unary operator

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