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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _addDefaultSkyKW(imageObjList):
 """Add MDRIZSKY keyword to "commanded" SCI headers of all input images, if that keyword does not already exist. """

    skyKW = "MDRIZSKY"
    Value = 0.0
    for imageSet in imageObjList:
        fname = imageSet._filename
        numchips=imageSet._numchips
        sciExt=imageSet.scienceExt
        fobj = fileutil.openImage(fname, mode='update', memmap=False)
        for chip in range(1,numchips+1,1):
            ext = (sciExt,chip)
            if not imageSet[ext].group_member:
                # skip over extensions not used in processing
                continue
            if skyKW not in fobj[ext].header:
                fobj[ext].header[skyKW] = (Value, 'Sky value computed by AstroDrizzle')
                log.info("MDRIZSKY keyword not found in the %s[%s,%d] header."%(
                            fname,sciExt,chip))
                log.info("    Adding MDRIZSKY to header with default value of 0.")
        fobj.close()