HCircularBillow:

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   Circular billow height model

   Since version 7.
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   Form:
   h = HCircularBillow( x, y, d )
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   ------
   Inputs
   ------
   x       (1,m)      x position
   y       (1,m)      y position
   d        (:)       Data structure
                      .b   (1,1)   Surface slope at outer edge
                      .r   (1,1)   Sail radius

   -------
   Outputs
   -------
   h       (1,m)

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   Reference:  Generalized Models for Solar Sails, L. Rios-Reyes and
               D. J. Scheeres, AAS 04-286
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