In this expression, r stands for any positive number. A program is such a rule. It is a rule that tells us and the computer how to produce data from some other data. Large programs consist of many small programs and combine them in some manner. It is therefore important that programmers name each rule as they write it down.
Programs combine basic operations.
Once we have defined a program, we may use it as if it were a primitive operation. For each variable listed to the right of the program name, we must supply one input. Many programs consume more than one input. Let us call these unknown numbers outer and inner . The rest is plain arithmetic.