prtol(t) sets the tolerance for printing zero to the absolute value of its
	 scalar argument t and returns the former value of the tolerance.
	 prtol with no arguments returns the current value of the tolerance.
	 When arrays are printed, values less than prtol in absolute value are
	 printed as zero.  prtol(0) turns off this effect so all values are
	 always printed no matter how small.