DHCWDX:

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   Computes the derivative of the chordwidth with respect to the 
   vector [ right ascension  ]
          [ declination      ]
          [ cant angle bias  ]
          [ earth width bias ] 

   The output is
   [w/ra, w/dec, w/cantAngleBias, w/earthWidthBias]
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   Form:
   H = DHCWDX( ra, dec, cg, sg, er, ue )
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   Inputs
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   ra                     Right ascension
   dec                    Declination
   cg                     Cosine of the cant angle
   sg                     Sine of the cant angle
   er         (n)         Angular earth radius
   ue         (3,n)       Nadir vector

   -------
   Outputs
   -------
   H                      Derivative

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