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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 pca(adata, **kwargs) -> Union[Axes, List[Axes], None]: """\ Scatter plot in PCA coordinates. Parameters {adata_color_etc} {scatter_bulk} {show_save_ax} Returns ------- If `show==False` a :class:`~matplotlib.axes.Axes` or a list of it. """

    return plot_scatter(adata, 'pca', **kwargs)