First of all, terminals are not weighted in GeneXproTools, that is, they are all weighted equally, no matter whether they are being chosen for the heads or the tails. So the only way of weighting certain variables would be either by duplicating them in the datasets or creating UDFs returning a certain terminal.

In the heads, however, terminals are chosen together with functions and, given that in version 4.0 there are no limits concerning the number of variables, it became essential to automatically balance the function set. This is documented in the Online Knowledge Base and was optimized to produce an efficient evolution. The rule of thumb is very simple: when the number of functions in the function set is smaller than the number of terminals, for each position in the heads, the probability of it being a function is 1/2. When functions outnumber terminals though, all elements (functions and terminals) are equally weighted. These rules are operational both during the creation of the initial population and during mutation. Also important is that when you are using random numerical constants, the rule of thumb for choosing the terminal “?” (the special terminal that represents the random numerical constants) both for the heads and tails is 1 out of 3 terminals.