The constraint sum 
should get
smaller with each step. If it does not, then see if the 
is
changing. If it is not, then the fit is near a solution,
and a directed step should be tried
(see Section A.1.8). Otherwise, the step is `cut' by
a fraction 
and retried. That is, instead of stepping to
, step to 
, defined by
or
Also,
Now to find the . At , 
,
the is
Define
Then,
Now, using (A.20), (A.27), (A.26), and (A.25),
Since the step may have to be cut multiple times, note that
