private ExpressionNode toExpressionNode(TensorFunction<Reference> f) {
        if (f instanceof ExpressionTensorFunction etf)
            return etf.expression;
        else
            return new TensorFunctionNode(f);
    }
Returns the tensor function wrapped by this
searchlib/src/main/java/com/yahoo/searchlib/rankingexpression/rule/TensorFunctionNode.java