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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 item_frequency(sa, xlabel=LABEL_DEFAULT, ylabel=LABEL_DEFAULT, title=LABEL_DEFAULT):
 """ Plots an item frequency of the sarray provided as input, and returns the resulting Plot object. The function supports SArrays with dtype str. Parameters sa : SArray The data to get an item frequency for. Must have dtype str xlabel : str (optional) The text label for the X axis. Defaults to "Values". ylabel : str (optional) The text label for the Y axis. Defaults to "Count". title : str (optional) The title of the plot. Defaults to LABEL_DEFAULT. If the value is LABEL_DEFAULT, the title will be "<xlabel> vs. <ylabel>". If the value is None, the title will be omitted. Otherwise, the string passed in as the title will be used as the plot title. Returns ------- out : Plot A :class: Plot object that is the item frequency plot. Examples -------- Make an item frequency of an SArray. """

    if (not isinstance(sa, tc.data_structures.sarray.SArray) or
        sa.dtype != str):
        raise ValueError("turicreate.visualization.item_frequency supports " + 
            "SArrays of dtype str")
    title = _get_title(title)
    plt_ref = tc.extensions.plot_item_frequency(sa, 
      xlabel, ylabel, title)
    return Plot(plt_ref)