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   Interface to dhcwdx. 
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   Form:
   H = DHCWDXi( x, u )
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   ------
   Inputs
   ------
   x             State vector [ra, dec, dcant, dearthRadius, bias]
   u             Auxiliary vector [cant;er;ue]

   -------
   Outputs
   -------
   H             Jacobian of the chordwidth

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