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   Find the surfaces that are in shadow. 
   The s vector is the shadow regions. The first two are for alpha, the second 
   for beta. Each surface can have two alpha limits and one beta limit.
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   Form:
   k = Shadow( sunAlpha, sunBeta, s )
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   Inputs
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   sunAlpha    (1,1)   Alpha angle of sun (orbit plane from +x)
   sunBeta     (1,1)   Beta angle of sun (out of plane)
   s           (6,n)   Limits for each surface [alpha1Min alpha1Max alpha2Min
                       alpha2Max betaMin betaMax]
 
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   Outputs
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   k                   Set of surfaces in shadow

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