anchor.diff:    }
   }
   
  +/// A material design circular progress indicator.
  +///
  +/// A widget that shows progress along a circle. There are two kinds of circular
  +/// progress indicators:
  +///
  +///  * _Determinate_. Determinate progress indicators have a specific value at
  +///    each point in time, and the value should increase monotonically from 0.0
  +///    to 1.0, at which time the indicator is complete. To create a determinate
  +///    progress indicator, use a non-null [value] between 0.0 and 1.0.
  +///  * _Indeterminate_. Indeterminate progress indicators do not have a specific
  +///    value at each point in time and instead indicate that progress is being
  +///    made without indicating how much progress remains. To create an
  +///    indeterminate progress indicator, use a null [value].
  +///
  +/// See also:
  +///
  +///  * [LinearProgressIndicator]
  +///  * <https://www.google.com/design/spec/components/progress-activity.html#progress-activity-types-of-indicators>
   class CircularProgressIndicator extends ProgressIndicator {
  +  /// Creates a circular progress indicator.
  +  ///
  +  /// The [value] argument can be either null (corresponding to an indeterminate
  +  /// progress indcator) or non-null (corresponding to a determinate progress
  +  /// indicator). See [value] for details.
     CircularProgressIndicator({
       Key key,
       double value
  
positive.code:       canvas.drawPath(path, paint);
  
      } else {
        // Non-determinate
        paint.strokeCap = StrokeCap.square;
  
        double arcSweep = math.max(headValue * 3 / 2 * math.PI - tailValue * 3 / 2 * math.PI, _kEpsilon);
  
negative.code: 
  // TODO(hansmuller): implement the support for buffer indicator
  
  /// A base class for material design progress indicators
  ///
  /// This widget cannot be instantiated directly. For a linear progress
  /// indicator, see [LinearProgressIndicator]. For a circular progress indicator,
  /// see [CircularProgressIndicator].
  ///
  /// See also:
  ///
  ///  * <https://www.google.com/design/spec/components/progress-activity.html>
  abstract class ProgressIndicator extends StatefulWidget {
    /// Creates a progress indicator.
    ///
    /// The [value] argument can be either null (corresponding to an indeterminate
    /// progress indcator) or non-null (corresponding to a determinate progress
    /// indicator). See [value] for details.
    ProgressIndicator({
      Key key,
      this.value
    }) : super(key: key);
  
    /// If non-null, the value of this progress indicator with 0.0 corresponding
    /// to no progress having been made and 1.0 corresponding to all the progress
    /// having been made.
    ///
    /// If null, this progress indicator is indeterminate, which means the
    /// indicator displays a predetermined animation that does not indicator how
    /// much actual progress is being made.
    final double value;
  
    Color _getBackgroundColor(BuildContext context) => Theme.of(context).backgroundColor;
    Color _getValueColor(BuildContext context) => Theme.of(context).primaryColor;
  