NAME:
    deriver
 PURPOSE: (one line)
    Compute the step-wise derivative of a function of one variable.
 DESCRIPTION:
      Calculates the step-wise derivative of a function by computing
  deltaf / deltax.  This requires that f and x be vectors with at least
  two elements.
      The uncertainties associated with the dependent variable of the
  function may be supplied via a keyword, in which case the uncertainties
  associated with the derivative are returned to the caller via another
  keyword parameter.
 CATEGORY:
    Mathematical
 CALLING SEQUENCE:
    deriver, x, f, newx, fprime, [, FERR=in_ferr, FPERR=out_fperr]
 INPUTS:
    x    : Independent variable.
    f    : Dependent variable.
 OPTIONAL INPUT PARAMETERS:

 KEYWORD PARAMETERS:
    FERR   = Uncertainty in f (optional input).
    FPERR  = Uncertainty in fprime (optional output).
 OUTPUTS:
    newx   : The centered x-values corresponding to fprime.
    fprime : The computed derivative of the dependent variable.
 COMMON BLOCKS:

 SIDE EFFECTS:

 RESTRICTIONS:

 PROCEDURE:

 MODIFICATION HISTORY:
    Written by Doug Loucks, Lowell Observatory, July, 1993.
  1999 Nov 24, Marc W. Buie, Lowell Observatory, changed name to eliminate
    a naming conflict with DERIVE, an IDL built-in routine.