/*
COMBINATORIAL EXAMPLE: a farmer has some chickens and cows for a total of 30 animals, the animals have 74 legs in total.
How many chickens does the farmer have?
*/
?- Chickens + Cows #= 30,
| Chickens * 2 + Cows * 4 #= 74,
| Chickens in 0..sup,
| Cows in 0..sup.
% Chickens = 23,
% Cows = 7.