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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 _varLib_finder(source, directory="", ext="ttf"):
 """Finder function to be used with varLib.build to find master TTFs given the filename of the source UFO master as specified in the designspace. It replaces the UFO directory with the one specified in 'directory' argument, and replaces the file extension with 'ext'. """

    fname = os.path.splitext(os.path.basename(source))[0] + "." + ext
    return os.path.join(directory, fname)