MFIT /STEP

          [SIC\]MFIT Y=f(X,&a,...) /STEP A2 B2 ...

    A2 B2 ... are used to pass the unity vectors (steps) for  the  iteration
    on  parameters  &A  &B (defaulted to 0.25). Poor choice of initial steps
    may lead to non convergence. Too small steps will  not  help  converging
    when  one  starts too far from the solution. Too large steps may lead to
    incorrect evaluation of the parameter errors. The optimal step  for  the
    determination  of the errors is about the error bar, so that all parame-
    ters become dimensionless.