A First Example

Let us follow the steps of development of a simple program and thereby explain some of the fundamental concepts of programming and of the basic facilities of Modula. The task shall be, given two natural numbers x and y, to compute their greatest common divisor (gcd). The mathematical knowledge needed for this problem is the following: 1.if x equals y, x (or y) is the desired result2.the gcd of two numbers remains unchanged, if we replace the larger number by the difference of the numbers, i.e. subtract the smaller number form the larger one.