Question: Plotting Removable Discontinuities

#1 Typically, on plots, removable discontinuities are represented by a small cicle at the discontinuity. How can such an object (i.e. the circle) be generated, preferrably automatically, by Maple at these points. #2 Also, if, for example, the signum function is plotted (i.e. a step discontinuity at x=0). How is an open circle plotted at (0,-1) and a closed circle plotted at (0,1)? Thanks in advance.
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