3.20 Define each of the following as constraint satisfaction problems:

a. The cryptarithmetic problem.

b. The channel-routing problem in VLSI layout.

c. The map-coloring problem. In map-coloring, the aim is to color countries on a map using a given set of colors, such that no two adjacent countries are the same color.

d. The rectilinear floor-planning problem, which involves finding nonoverlapping places in a large rectangle for a number of smaller rectangles.