A complex number $z$ is said to be unimodular if $|z|=1.$ Suppose $z_1$ and $z_2$ are complex numbers such that $\dfrac{z_1-2z_2}{2-z_1\bar{z_2}}$ is unimodular and $z_2$ is not unimodular. Then the point $z_1$ lies on a :

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