anchor.diff:  
   (defparameter *b* 1.5d0)
   (defparameter *a* 1.1d0)
  +
   (defun y (x coeffs)
     (let ((a (grid:aref coeffs 0))
   	(b (grid:aref coeffs 1)))
  
positive.code: 	   (gsll:iterate fit)
  	   (setf covariance (gsll:ls-covariance fit 0.0d0 covariance))
  	   (when print-steps
  	     (format t "iter: ~d x = ~15,8f ~15,8f |f(x)|=~7,6g~&"
  		     (1+ iter) (fitx 0) (fitx 1)
  		     (norm-f fit)))
  	 finally
  	   (let* ((chi (norm-f fit))
  
negative.code:     (macrolet ((fitx (i) `(grid:aref (gsll:solution fit) ,i))
  	       (err (i) `(sqrt (grid:aref covariance ,i ,i))))
        (when print-steps
  	(format t "iter: ~d x = ~{~15,8f ~} |f(x)|=~7,6g~&"
  		0 (loop for i below n-coeffs
  		       collect (fitx i))
  		(norm-f fit)))
        (loop for iter from 0 below max-steps
  	 until
  