1. Consider a user having already entered a litte formula like

x² + 2 x + 5 over 5

Imagine he would like to add some square root (the user has not think of it
before).  He select "2 x + 5" in the text box editor beneath the formula and
then use the button "square root" in the selection window.

Well, he get 

x² + sqrt {<?>} over 5 

instead of 

x² + sqrt { 2 x + 5 } over 5

2.  Solving the problem of this user allows also to type formulas using only the
mouse.  Simply double clicking on elements we want to decorate and then choosing
the decoration in the selection (decoration?) windows.  

3. What should happend?  proposed solution : <?> in the text of the
decorations/inserted formulas should be replace by current selection and not the
current selection replaced by the decoration/inserted formula.

4. Of course, in case of having several <?> there would be a problem of choosing
which <?> is the more user friendly replaced by current selection but may be
this is an other story...