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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 append_to_path (path, directory):
 """Add a directory to the PATH environment variable, if it is a valid directory."""

    if not os.path.isdir(directory) or directory in path:
        return path
    if not path.endswith(os.pathsep):
        path += os.pathsep
    return path + directory