GSREDUCE in three stages:

1) overscan subtract, bias subtract, trim.
    => @objects.lis: fl_over+ fl_trim+ fl_bias+ ovs_fli+ bias=gS20070913S0213_bias.fits
2) flatfield
    => @objects_a.lis: fl_flat+ flat=flat_a
    => @objects_b.lis: fl_flat+ flat=flat_a
3) mosaic, fix pixels, gsappwave
    => fl_gmos+ fl_fixpix+ fl_gssap+ fl_cut+

Update prefixes each time!

Note (repeat from gsflat): There is some debate as to whether it is better to bias subtract and/or overscan subtract. 
Gemini-N biases have structure so it's important to bias subtract Gemini N data. Gemini S doesn't have this structure. 
There are differing opinions about overscan subtracting the flat in addition to bias subtracting. Some at the Gemini Help 
desk have strongly recommended overscan subtracting ONLY for Gemini S data, and doing both bias and overscan subtracting 
for Gemini N, whereas others recommend against using overscan subtracting at all because of light contamination, which is 
speculated to affect brighter targets more than fainter objects. As of now, there is no concensus on this subject, but 
we've chosen to do both.

PACKAGE = gmos
   TASK = gsreduce

     inimages = "@objects_a.lis" Input GMOS images or list
     (outimages = "")             Output images or list
     (outpref = "gs")           Prefix for output images
     (fl_over = yes)             Subtract overscan level
    *(fl_trim = yes)             Trim off the overscan section
    *(fl_bias = yes)             Subtract bias image
     (fl_gscrrej = no)             Clean images for cosmic rays
     (fl_dark = no)             Subtract (scaled) dark image
    *(fl_flat = no)            Apply flat field correction
    *(fl_gmosaic = no)            Mosaic science extensions
    *(fl_fixpix = no)            Interpolate across chip gaps if mosaicing
    *(fl_gsappwave = no)            Run gsappwave on reduced image
    *(fl_cut = no)            Cut slits into separate spectra if mosaicing
     (fl_title = yes)            Put object id in title of cut spectra (MOS only)
     (fl_vardq = no)             Create variance and data quality frames
     (bias = "biasfile")    Bias image name
     (dark = "")             Dark image name
    *(flatim = "flat_a")       Flatfield (output of GSFLAT) image