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                \begin{eqnarray*}
                \textbf{Arg}(z)= \left\{\def\arraystretch{1.2}%
                \begin{array}{@{}c@{\quad}r@{}}
                \arctan \frac{y}{x} &amp; \;\text{if $x&gt;0,$ $y\in \mathbb R$}\\
                \arctan \frac{y}{x}+\pi &amp; \;\text{if $x &lt;0,$ $y\geq 0$}\\
                \arctan \frac{y}{x}-\pi &amp; \;\text{if $x &lt; 0,$ $y &lt; 0$}\\
                \frac{\pi}{2} &amp; \;\text{if $x=0,$ $y&gt; 0$}\\
           