Derivative dy/dz=-i?

[Tom83B]

There's this text that is supposed to help us with some problems in one competition (I could send the link, but it's pdf in czech...) and there's written, that y_{,z}=-i. It's about complex numbers so the z is probably a complex number, but I can't see why the derivative should be -i...

[mathman]

Guessing: z = x + iy, dz/dy = i, dy/dz = 1/i = -i.

[lurflurf]

It needs to be stated what is being held constant in the derivative
for example we might write
y=-i(z-x)=(-i/2)(z-z*)
but
[-i(z-x)]z=-i
while
[(-i/2)(z-z*)]z=-i/2