public String expression(){
         
         String word;
         switch(randomFunc){
             case 0: 
                 word = "sin(" + nextNode.expression()+ ")" ; 
                 break;
             case 1:
                 word = "cos(" + nextNode.expression()+ ")" ; 
                 break;
             case 2:
                 word = "avg(" + nextNode.expression() + ","+ nextNode.expression() + ")"; 
                 break;
             default:
                 word = "";
                 break;
         }
         return word;
     }