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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 verifyRefimage(refimage):
 """ Verify that the value of refimage specified by the user points to an extension with a proper WCS defined. It starts by making sure an extension gets specified by the user when using a MEF file. The final check comes by looking for a CD matrix in the WCS object itself. If either test fails, it returns a value of False. """

    valid = True

    # start by trying to see whether the code can even find the file
    if is_blank(refimage):
        valid=True
        return valid

    refroot,extroot = fileutil.parseFilename(refimage)
    if not os.path.exists(refroot):
        valid = False
        return valid

    # if a MEF has been specified, make sure extension contains a valid WCS
    if valid:
        if extroot is None:
            extn = findWCSExtn(refimage)
            if extn is None:
                valid = False
            else:
                valid = True
        else:
            # check for CD matrix in WCS object
            refwcs = wcsutil.HSTWCS(refimage)
            if not refwcs.wcs.has_cd():
                valid = False
            else:
                valid = True
            del refwcs

    return valid