RHSZermelo:

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   Zermelo's differential equation right hand side.
   Models a constant velocity field.

   Since version 7.
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   Form:
   vargout = RHSZermelo( t, x, flag, d )
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   Inputs
   ------
   t              (1,1) Time
   x              (4,1) State [x;y;lambdaX;lambdaY]
   flag           (1,1) Flag from ode 113
   d              (1,1) Data structure
                        .h (1,1) Characteristic dimension
                        .v (1,1) Costate flag

   -------
   Outputs
   -------
   xDot           (4,1) State derivative

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   Reference: Bryson, Ho, "Applied Optimal Control", Hemisphere Publishing. 
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