In a quiz, I was asked to find all of the combinations of organic molecule of $\ce{C4H6O2}$. The combination excludes -COOH, -OH, double bonds and the alkoxy group.
The only problem, as far as I am concerned, is the definition of the term "alkoxy group". According to this Wikipedia article, a Peroxy or Methelenedioxy should be one of the possible combinations allowed. However, the names do have the "-oxy" suffix in them. So I am uncertain whether to include Peroxy's and Methelenedioxy's in the list.
P.S. Below is a picture of the Methelenedioxy group and the peroxy group, respectively, from the already mentioned Wikipedia article.