def _appendTrlFile(trlfile,drizfile):
    """ Append drizfile to already existing trlfile from CALXXX.
    """
    if not os.path.exists(drizfile):
        return
    # Open already existing CALWF3 trailer file for appending
    ftrl = open(trlfile,'a')
    # Open astrodrizzle trailer file
    fdriz = open(drizfile)

    # Read in drizzle comments
    _dlines = fdriz.readlines()

    # Append them to CALWF3 trailer file
    ftrl.writelines(_dlines)

    # Close all files
    ftrl.close()
    fdriz.close()

    # Now, clean up astrodrizzle trailer file
    os.remove(drizfile)