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   Compute the desired phase on the circle from the desired phase on the ellipse.
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   Form:
   alpha0 = CirclePhase( beta0 );
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   Inputs
   ------
   beta0            (1)      desired phase angle at equator crossing   [rad]

   -------
   Outputs
   -------
   alpha0           (1)     phase angle on circle such that
                               the y-component of the circle
                               is equal to that of the ellipse        [rad]

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