Sketch Block: Undo/Redo

Clicking the undo button completely wipes the entire drawing. When I sketch, I sometimes draw a line a few times before I'm satisfied with how it looks, but with this block I have no choice but to clear the entire drawing and start over. Making any mistake whatsoever means goodbye to the entire sketch.

I'd like to use this block to make small doodles or even basic diagrams directly from my phone (where I do most of my blogging) to help explain something without having to resort to using another program and uploading a raster image. This is difficult without an eraser tool, but basically impossible without at least a line-by-line undo function.

I feel like this block has the potential to completely change how I blog!