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   Finds the vectors
   rho   = z - h
   dh/dx = H
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   Form:
   [rho, H, W, JL] = FX( x )
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   Inputs
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   x         State vector

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   Outputs
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   rho       Residual vector z - h
   H         dh/dx 
   W         The measurement weighting vector
   JL        Cost eliminated by eliminating measurements
     
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