Just drag the pen-library sprite into your backpack =) From there drag it into your project and now you can use the custom blocks. There's a script at the top-right which draws the pencil and the lines. You can delete it.
A pen library wich lets you draw various shapes like circles or filled rectangles. Free to use! Any ideas or suggestions are welcome! Custom blocks with a "_"-prefix like "_tri" are system interior custom blocks and are not supposed to be used by you =)
