Photoshop's pencil tool has a setting for "auto erase". When set, clicking on a pixel of the current foreground color causes it to draw over it with the background color. This is great if you're drawing on a colored background, but useless when the background is transparent.
How can I set the pencil tool to erase pixels (replace them with transparent) instead of just drawing the background color over them? Or if this is not possible, is there another tool that can be set to draw a single pixel in the foreground color and then auto erase to transparent?