Assume a function is real-valued

edit

In the following expression:

var('z')

u = function('u')(z)
v = function('v')(z)

f = u + i * v

show(f.diff(z).conjugate().expand().simplify_full())
# conjugate(diff(u(z), z)) - I*conjugate(diff(v(z), z))

I would like Sage to make the simplification conjugate(diff(u(z), z)) = diff(u(z), z) because $u, v$ are intended to be real-valued. But my first attempt:

assume(u, 'real')

didn't work:

TypeError: self (=u(z)) must be a relational expression

What is the right way to write this assumption?