PULSE(strtme,endtme)

Function whose value is 1.0 when the step time (i.e. variable t) is
greater than or equal to 'strtme' and less than or equal to 'endtme'.
That is,

    PULSE = 1  when  strtme <= t <= endtme.

Larger pulses can be achieved by multiplying PULSE times a constant.
E.g. 10 * PULSE(0,.001).

PULSE can be used to enable/disable an expression at different times
in the processing domain.  For example,

    fx = expr1*PULSE(-1000,0) + expr2*PULSE(0,1000);
