Why Ants are
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The artificial ant problem [Koza1992, pages 147--155,]
is a well studied problem
often used as a GP benchmark.
Briefly the problem is to devise a program which can successfully
navigate an artificial ant along a twisting trail on a 
toroidal
grid.
The program can use three operations, Move, Right and Left, to move the
ant forward one square, turn to the right or turn to the left. Each of
these operations takes one time unit.
The sensing function IfFoodAhead looks into the square the ant is
currently facing and then executes one of its two arguments depending
upon whether that square contains food or is empty.
Two other functions, Prog2 and Prog3, are provided.
These take two and three arguments respectively
which are executed in sequence.
The artificial ant must follow the ``Santa Fe trail'', which consists of 144 squares with 21 turns. There are 89 food units distributed non-uniformly along it. Each time the ant enters a square containing food the ant eats it. The amount of food eaten is used as the fitness measure of the control program.
The fitness function, function and terminal sets etc.\ we use are identical to [Langdon and Poli1997a] cf. Table 1.
