COMMAND NAME:

        DELTA_TIME

USAGE:
        DELTA_TIME
        DELTA_TIME <time1>
        DELTA_TIME <time1> <time2>

        This command prints the elapsed time between two values.

        The two values are determined by a puck poke with the LEFT MOUSE
        BUTTON or an integer typed at the keyboard. If the user pokes a 
        signal a marker and its value will snap to the nearest edge.
        The command is iterative so that whilst the LEFT MOUSE BUTTON
        is poked the delta time is between the current and the last poke.
        If the RIGHT MOUSE BUTTON is poked the reference time can be
        repositioned.
