Constants used by 
    
    
    
    
    
    
    
    
    
    
    
    
Clock#tick to determine behavior
when Clock#startTime or Clock#stopTime is reached.
    Members
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    WhenClock#startTimeorClock#stopTimeis reached,Clock#tickwill not advanceClock#currentTimeany further.
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    WhenClock#stopTimeis reached,Clock#tickwill advanceClock#currentTimeto the opposite end of the interval. When time is moving backwards,Clock#tickwill not advance pastClock#startTime
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    Clock#tickwill always advances the clock in its current direction.
