Using the Art Eraser tool
The Art Eraser tool 
lets you erase art media from an image. While this tool is not strictly realistic, it does provide you with the flexibility to erase mistakes, rather than undoing entire strokes.
Art Eraser tool options
Art Eraser tool options include
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Shape — defines the shape of the eraser tip
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Size — defines the size of the tool head in pixels
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Thickness — defines the aspect ratio of the tool head
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Rotation — defines the angle of head rotation in degrees
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Head tracking — controls whether the brush head bends around the path of the stroke, or whether it remains at a fixed angle