anchor.diff:  
       if (type == EvaluationType.VECTOR) {
         //TODO apply function to all vector elements
  -      throw new UnimplementedError('Can not evaluate functions on vectors yet.');
       }
   
       if (type == EvaluationType.INTERVAL) {
  
positive.code:    * You also can use variables or arbitrary expressions:
     *     x = new Variable('x');
     *     exp = new Exponential(x);
     *     exp = new Exponential(x + 4);
     */
    Exponential(exp): super._unary("exp", exp);
  
  
negative.code: 
      if (type == EvaluationType.VECTOR) {
        //TODO apply function to all vector elements
      }
  
      if (type == EvaluationType.INTERVAL) {
  