public class Const extends Leaf {
  
    double value;
    
    public Const(double value) {
	     this.value = value;
    }
    
    public Double eval(Assignation ass) {
	     return value;
    }   
    public Node derive(String var) {
	     return new Const(0);
    }
 
    public Node substitute(String var, double d) {
	     return this;
    }
}