Simple masking tool
To make sharp edges when shading or using other fuzzy tools. The tool would be like the hard eraser not erasing but making the mask. Clear mask mode is needed also. Or clear all masks option. The mask itself is visible in some color like red and should be see-through. Thanks!
look for instance: http://www.craftedblog.com/2011/04/masking-fluid-is-awesome.html