\noindent\qquad $\overline{\overline x} = x$, $\overline{x\pm y} = \overline{x} \pm \overline{y}$, $\overline{x\cdot y} = \overline x \cdot \overline y$, $x\cdot \overline x \in {\bb R}$.
\noindent\qquad $\overline{\overline x} = x$, $\overline{x\pm y} = \overline{x} \pm \overline{y}$, $\overline{x\cdot y} = \overline x \cdot \overline y$, $x\cdot \overline x \in {\bb R}$.