I was reading "Optical Recognition Systems" book and in chapter 3.3.2.3 where methods for full genetic programming are described, a paper is describing an ADF (automatically defined functions) like this:
The ADFs have the terminal set Tf ={X, N, S, W, E, NE, NW, SE, SW} and the function set Ff = {AND, OR, NOT}. The main program has the terminal set Tc = {I, L, NIL} and a set of functions (Fc) containing movement functions, logical operators, four ADFs and the HOMING operator.
I am confused about the meaning of HOMING operator. An explanation or some links to point me in the right direction would be great.