Write for-each. [2.23]
Merge branch 'ex2.23'
Write deep-reverse. [2.27]
Merge branch 'ex2.27'
Write fringe. [2.28]
Merge branch 'ex2.28'
Mobile list implementation from book. [2.29]
Write selectors. [2.29]
Write total-weight. [2.29]
Write pair representation. [2.29]
Merge branch 'ex2.29'