anchor.diff:    int get domainDimension => args.length;
   
     String toString() => '$name($args)';
  -  
  +
     /**
      * Returns the full string representation of this function.
      * This could include the name, variables and expression.
  -   * 
  +   *
      * Any subclass should decide whether to override this method.
      */
     String toFullString() => toString();
  
positive.code:    * Simplify argument.
     */
    Expression simplify() {
      new Root(n, arg.simplify());
    }
  
    evaluate(EvaluationType type, ContextModel context) {
      throw new UnimplementedError('Can not evaluate n-th root yet.');
    }
  
  
negative.code:     }
  
      if (type == EvaluationType.VECTOR) {
        //TODO apply function to all vector elements
        throw new UnimplementedError('Can not evaluate functions on vectors yet.');
      }
  
  