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Some days ago I was developing a library to show and run mathematics exercises on the web, and I needed a simple calculator widget.
A search in the mootools forge led to 0 results, so I decided to write my own plugin, also because I needed only some basics operations (sum, difference, multiplication, division, percentage, pow and sqrt).
Such plugin let's you create a calculator instance in a specified container or over the document, and make it draggable. Methods are provided in order the hide/show it or destroy it.
The implementation was very very "fast" so the code can be and probably will be enhanced in the next future.