Adding exercises 21 and 22 and examples for lecture 21
Merge branch 'master' of https://bitbucket.org/sathrumlu/pa2sp14
Adding recursive examples
Merge branch 'master' of https://bitbucket.org/sathrumlu/pa2sp14
Adding exercise 23 and 24
Merge branch 'master' of https://bitbucket.org/sathrumlu/pa2sp14
"committing large batch"