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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 preload_top(meta):
 """Load one topology file into memory. This function checks to make sure there's only one topology file in play. When sampling frames, you have to have all the same topology to concatenate. Parameters meta : pd.DataFrame The DataFrame of metadata with a column named 'top_fn' Returns ------- top : md.Topology The one topology file that can be used for all trajectories. """

    top_fns = set(meta['top_fn'])
    if len(top_fns) != 1:
        raise ValueError("More than one topology is used in this project!")
    return md.load_topology(top_fns.pop())