In gp: I have a boolean function GreaterThan that I use to compare two variables, but I want to make sure that I am not comparing the same two variables. In other words, > var1 var2 is ok, and > var1 var2 is ok, but > var1 var1 is not ok, and > var2 var2 is not ok either. Here is the node constraint for the boolean function (from params file): gp.nc.4 = ec.gp.GPNodeConstraints gp.nc.4.name = ncBool-Child1Nil-Child2Nil gp.nc.4.returns = bool gp.nc.4.size = 2 gp.nc.4.child.0 = nil gp.nc.4.child.1 = nil Is there a way to restrict this behavior in the params file? The children will be of the same type, but can't be the same variable. Or do I have to do something in the checkConstraints method of the GPNode? Shane