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   Computes the derivatives of the dihedral angle with respect
   to right ascension and declination
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   Form:
   H = DHDADX( ra, dec, us, ue )
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   Inputs
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   ra                     Right Ascension
   dec                    Declination
   us           (3,n)     Unit sun vectors
   ue           (3,n)     Unit nadir vectors

   -------
   Outputs
   -------
   H            (n,2)     [�da/�ra �da/�dec]

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