==> logic/river.crossing.p <==
Three humans, one big monkey and two small monkeys are to cross a river:
 	a) Only humans and the big monkey can row the boat.
 	b) At all times, the number of human on either side of the
 	   river must be GREATER OR EQUAL to the number of monkeys
 	   on THAT side. ( Or else the humans will be eaten by the monkeys!)