A compass tool, that lets you draw circles or sections of a circle. This tool is much needed in math and physics class.

(The app "Digital exercise book" has this feature, and once you figure out how it works, this feature is awesome. Apart from the compass and set square tools, Squid is way better for writing, though. That's why I want this feature in Squid.)

Another way of realizing a compass tool would be to have a transparent disc with a dot in the center and a handle near the outer line. You could grab the disc near the center and place it onto your center point of the circle you want to draw. Then you can adjust the diameter by grabbing the handle on the outer line and moving it inwards or outwards. The diameter and radius are displayed somewhere, maybe in a floating box near the disc. Then you can use the pen tool to draw around the disc and make circles or sections of circles.
You could also click the box, to set the diameter or radius by typing them in.