The famous computer scientist, Edsger Dijkstra, made the following observations for m and n >= 1:
1. If m = n, then the greatest common divisor of m and n is m.
2. If m > n, then the greatest common divisor of m and n is the greatest divisor of m-n and n.
Based on these observations, implement a function to compute the greatest common divisor of two given numbers >= 1.
Follow all the steps of the design recipe. and write the termination argument and check-expect.