Exercise 1.19 - fast-fib-iterative, just the same thing again, but with 2x2 matrices, and iterative ONLY
removed dead-end code, to keep trunk clean
Section 1.2.5 - Euclid's Algorithm, which I still find to be non-trivial
Exercise 1.20 - using Euclid's Algorithm to illustrate how inefficient normal-order evaluation is when recursing with nested function calls