gen0 - Normalize a function to lie within the range +max to -max:

	gen T gen0 F   max ;

F is a previously defined function with a maximum absolute value X.
Gen0 scales this function so that the old value X corresponds to max.
If max is omitted, it is assumed to be 1.0.  Some functions consist of
only positive values, or negative values (such as an amplitude
envelope), or both positive and negative values (such as a waveform).
Gen0 will insure that no value exceeds max in either case.  Examples:

gen 0 gen0 f1 1;	(Normalizes maximum value of f1 to 1.0)
gen 0 gen0 f2 1000Hz; (Normalizes f2 so that its maximum value corresponds to
			1000 Hz)
