more elaborate code example

[BMaxV]

I found the code example not very explanatory.

Also the input of (Left, right, bottom, top) is counter intuitive to me and should be on some list for when a new GUI framework is created. It should be at least (Left, right, top, bottom) , I would prefer a canvas that's just defined with (x_length, y_length). That doesn't explicitly forbid using negative values. But that's probably going to break stuff.