code: public Primitive anyPrimitive(RandEngine randEngine)
     {
        double r = randEngine.uniform();
  
        if(r < 0.3333) {
           return constantSet.any(randEngine);
        } else if(r < 0.6666 && variableSet.size() > 0) {
           return variableSet.any(randEngine);
        } else {
           return anyOperator(randEngine);
        }
     }
docs: / <returns>The randomly selected primitive</returns>