HP-71B: 1^Inf Error

[Albert Chan]

To be more correct, sign(Im(acos(z))) = sign(-Im(z))

Here is Kahan's acos(z) implementation.
https://www.hpmuseum.org/forum/thread-13...#pid151453

lua> z = I.new(2,0)
lua> a = I.sqrt(1+z)
lua> b = I.sqrt(1-z)
lua> x2 = atan(b:real() / a:real()) * 2
lua> y2 = asinh(a:real()*b:imag() - a:imag()*b:real())
lua> I.new(x2, copysign(y2, -z:imag()))      -- = acos(2+0j)
(0-1.3169578969248166*I)