NAME:
  wfpc2_distorted
 PURPOSE:   (one line only)
  Compute inverse of the wfpc2\_metric function.
 DESCRIPTION:
  Iterative reverse solution of wfpc2_metric (HST WFPC2 camera distortion
    model).  This depends entirely on the solutions in the wfpc2_metric
    program.
 CATEGORY:
  CCD data processing
 CALLING SEQUENCE:
  wfpc2_distorted,xundist,yundist,xdist,ydist
 INPUTS:
  xundist - X coordinate in the undistorted (sky) plane
  yundist - Y coordinate in the undistorted (sky) plane
 OPTIONAL INPUT PARAMETERS:
  ITER - Number of iteration to run to get the answer (default=1)
  FILTER - String with the name of the filter used (this defines the
             distortion coefficients used.
 KEYWORD INPUT PARAMETERS:
 OUTPUTS:
  xdist   - X coordinate in the native chips coordinates
  ydist   - Y coordinate in the native chips coordinates
 KEYWORD OUTPUT PARAMETERS:
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
   Written by Peter Collins, Lowell Observatory, circa 2008
   2008/12/15, MWB, promoted internal calculations to double precision
   2010/03/08, MWB, placed into library with documentation