COMMAND NAME:

        REF_TIME

USAGE:
        REF_TIME
        REF_TIME <time1>
        REF_TIME <time1> <time2>

        This command prints the elapsed time between two values.
        It is different from DELTA_TIME command in that the time reported
	is always from the first point - i.e., the reference point remains
	same.
        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 time between the first poke and the latest poke is
	reported.
