	LETZ generates a data file of z values given an 
	expression in terms of x and y.  

	e.g. the following example does something like this:

		For y = 0 to 20 step 1
			For x = 0 to 30 step 1
				z[x,y] = x+sin(y^2)/pi+10.22 
			next x
		next y
	

----------- Log of a typical LETZ session ------------------------------
$ letz
File to store data in ? jack
Storing data in {jack.z} and commands in {jack.let} 
Enter xmin,xmax,xstep (on one line) [0,30,1] ? 
Enter ymin,ymax,ystep [0,30,1] ? 0,20,1
For y = 0 to 20 step 1
For x = 0 to 30 step 1
Valid funtions:  (trig functions use radians)
	abs(), atn(), cos(), exp(), fix(),
	int(), log(), log10(), rnd(), sgn(), sin(),
	sqr(), sqrt(), tan(), PI, (x^3 = x*x*x) 
	e.g.	3*x^2+sin(y*180/pi)
Enter equation:    
 ? x+sin(y^2)/pi+10.22
z = x+sin(y^2)/pi+10.22 

Y = 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 

Use   LETZ <jack.let   to remake the same data file
$ 
$ ----------- End of Log ------------------------------

	The file JACK.Z now contains the required data.

	The file JACK.LET contains

--------------JACK.LET-----------
jack.z
0, 30, 1
0, 20, 1
x+sin(y^2)/pi+10.22
-------------------end of file----

	This can be edited and then used to regenerate the data
	using a different range or a different equation.

