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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 out_name(stem, timestep=None):
 """Return StagPy out file name. Args: stem (str):
 short description of file content. timestep (int):
 timestep if relevant. Returns: str: the output file name. Other Parameters: conf.core.outname (str):
 the generic name stem, defaults to ``'stagpy'``. """

    if timestep is not None:
        stem = (stem + INT_FMT).format(timestep)
    return conf.core.outname + '_' + stem