HQuadrantBillow:

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   Models a single sail quadrant as a section of an oblique cone.
   Linear billow displacement along center of quadrant from inside corner to
   outside edge. Quadratic billow along this reference line.

   Since version 7.
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   Form:
   z = HQuadrantBillow( x, y, data )
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   Inputs:
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   x       (1,m)      x position
   y       (1,m)      y position
   data     (:)       Data structure
                      .b    % billow depth (fraction of boom length)
                      .L    % length along boom

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   Outputs
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   z       (1,m)

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