I'm reading a proof for the statement: let $G\subset\mathbb{C}$ be an open set containing all inner regions of piecewise horizontal or vertical closed linear curves in $G$, and let $u$ be harmonic on $G$. Then there exists a harmonic conjugate $v$ of $u$ on $G$. 

The proof goes on as follows.

Since $u$ is harmonic, it satisfies $\frac{\partial^2u}{\partial\overline{z}\partial z}=0$ on $G$, thus $g=\frac{\partial u}{\partial z}$ satisfies C-R. Define the holomorphic function $F(z)=\int\_a^z g(w)\,\mathrm{d}w$, where we integrate over a piecewise horizontal or vertical linear curve (it follows from Cauchy that it doesn't matter which one). Then $\frac{\partial F}{\partial z}=\frac{\partial u}{\partial z}=g(z)$. Then \begin{align\*}\frac{\partial }{\partial z}(F+\overline{F})&=\frac{\partial u}{\partial z};\\\frac{\partial }{\partial \overline{z}}(F+\overline{F})&=\frac{\partial u}{\partial \overline{z}}.\end{align\*} The last equation follows because $u$ is a real valued function. Thus $F+\overline{F}-u=C\in\mathbb{R}$, thus $u=2\mathrm{Re}(F)-C$, and we could take $v=2\mathrm{Im}(F)$. 

I can follow this proof, except for the step $\frac{\partial }{\partial z}(F+\overline{F})=\frac{\partial u}{\partial z}$. They mention that this is true because $\frac{\partial\overline{F}}{\partial z}=0$. I don't understand how we could just take the derivative of a complex conjugate of a function; also in general this step does not seem to make sense. Similar problems arise at the second line of the aligned part. I'm looking for some explanation for this step. Any help or reference is much appreciated!