2 GRADIENT
  Syntax: GRADIENT

  This command should not be used inside Mn_Fit. Indicates that your FCN
  routine is capable of calculating  first derivatives. (Means `IFLAG=2'
  will also be  used in addition to  the standard  `IFLAG=4'). They will
  then  be used  by  `MIGRAD'  (instead  of  by-hand  finite  difference
  quotients which may be  inaccurate). Mind  you, it really does not pay
  off. Probably you will  boil off a lot of  c.p.u. time without getting
  your money's worth  back. Difference  quotients are usually quite good
  (except  maybe  when  calculating  the  error  matrix, but  you'll run
  `MINOS' anyway, won't you???)

  NO_GRADIENT turns it off again.

