Clearing information from the track screen - attributes

In the tracking screen (select a channel, press "Track") is there a way to clear the information stored for attributes.  You can clear intensities by setting them to zero but this doesn't work with attributes.  Zero is actually a value where attributes are concerned and thus doesn't work to clear the information.  I'd like to be able to remove the values to restore the tracking, similar to @ Enter on the Obsession.

 Thanks,
Mat

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