NLEqSolver:

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   Solves a set of equations for zero using Newton's method.
   The function must be of the form

   fun( x, d )

   where d is a data structure with parameters

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   Form:
   [x, err, n] = NLEqSolver( fun, x, opt, d )
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   ------
   Inputs
   ------
   fun           (1,:)   String with the function name
   x             (:,1)   The initial solution guess
   opt           (1,1)   The options for the solver  
                         .nMax (1,1) Maximum number of iterations
                         .tol  (1,1) Tolerance on error
                         .dX   (1,1) Perturbation for the Jacobian
   d             (1,1)   Data structure passed to fun

   -------
   Outputs
   -------
   x             (:,1)   The solution
   err           (:,1)   The errors
   n             (1,1)   Number of iterations

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