For an all-pass system $H(z)= \frac{(z^{-1}-b)}{(1-az^{-1})}$, where $\mid H(e^{-j\omega }) \mid= 1,$ for all $\omega$. If $\text{Re}(a)\neq 0, \: \text{Im}(a)\neq 0,$then $b$ equals