Design and implement an applet that comprises a text output window,
in which text exercises are displayed. In this window, it is possible
to mark arbitrary pieces of text and assign them a meaning. For
instance, marking a piece of text green denotes that this part is a
given assumption, while a red marked text denotes the unknown. The
applet should be able to verify all marked pieces and provide feedback.
The applet should take any text exercise as input, i.e, there are
no hardcoded exercises. Define an input format. Find three nice
illustrating examples. (***)