Hi All,

     I am a new comer to the ECJ software who are studying Tutorial4 to 
run a Genetic Programming example on Symbolic Regression Problem. 

     In this example, I find that the "nil" GPType is used for the 
arguments and the return type of the "Add", "Sub" and "Mul" GPNodes. 

     Could I have some information on what this "nil" GPType stand for and 
could I define my own GPTypes for the GPNodeConstraints of these GPNodes ?

     Thanks in advance for any suggestion.