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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 boundary(ax, scale, axes_colors=None, **kwargs):
 """ Plots the boundary of the simplex. Creates and returns matplotlib axis if none given. Parameters ax: Matplotlib AxesSubplot, None The subplot to draw on. scale: float Simplex scale size. kwargs: Any kwargs to pass through to matplotlib. axes_colors: dict Option for coloring boundaries different colors. e.g. {'l': 'g'} for coloring the left axis boundary green """


    # Set default color as black.
    if axes_colors is None:
        axes_colors = dict()
    for _axis in ['l', 'r', 'b']:
        if _axis not in axes_colors.keys():
            axes_colors[_axis] = 'black'

    horizontal_line(ax, scale, 0, color=axes_colors['b'], **kwargs)
    left_parallel_line(ax, scale, 0, color=axes_colors['l'], **kwargs)
    right_parallel_line(ax, scale, 0, color=axes_colors['r'], **kwargs)
    return ax