Exercise 4.9: Many languages support a variety of
iteration constructs, such as do, for, while, and
until.  In Scheme, iterative processes can be expressed in terms of
ordinary procedure calls, so special iteration constructs provide no essential
gain in computational power.  On the other hand, such constructs are often
convenient.  Design some iteration constructs, give examples of their use, and
show how to implement them as derived expressions.