# Assume a function is real-valued

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?