I am going to design a textarea for writing some formula. Since typing entire formula may cause mistake in formula, I think its is better that users drag and drop variables from another panel. I believe that stackoverflow's tag option could be usefull for this purpose.  

[![enter image description here](https://i.stack.imgur.com/5K6iE.jpg)](https://i.stack.imgur.com/5K6iE.jpg)

How it is possible to design a text area like this?