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   Computes the dihedral angle for a spinning spacecraft.
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   Form:
   da = HDA( uA, uS, uE )
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   Inputs
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   uA                     Spacecraft unit vector
   uS           (3,n)     Unit sun vectors
   uE           (3,n)     Unit nadir vectors

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   Outputs
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   da           (n)       Dihedral angle

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