RADARCTAN num
(RADARCTAN x y)

	outputs the arctangent, in radians, of its input.  With two
	inputs, outputs the arctangent of y/x, if x is nonzero, or
	pi/2 or -pi/2 depending on the sign of y, if x is zero.

	The expression 2*(RADARCTAN 0 1) can be used to get the
	value of pi.

