"Vanilla" Ackermann function from original exercise [1.10]
Merge branch 'ex1.10'
Recursive implementation of f. [1.11]
Iterative implementation, after http://www.billthelizard.com/2009/11/sicp-exercise-111.html [1.11]
Merge branch 'ex1.11'
Pascal's triangle generator [1.12]
Merge branch 'ex1.12'