Block, Quote & Co. have finally discovered that XML's ``parentheses'' are really just like Scheme's parentheses. Now they want to invest in complicated functions that read/write XML and represent it in Scheme. Before they spend a lot of money, however, they want to study different forms of data representations. So they have hired knowledgeable Scheme experts from Northeastern's 211 course to transliterate XML into Scheme representations by hand; they also intend to use these hand-created representations in test cases later, which means they really need experts.

Here is their most important XML example:

  <blockquote>
    a
    b
    <blockquote>
     c 
     d 
    </blockquote>
    e
  </blockquote>

Represent this piece of XML into the following three data representations.