Select subexpression in equation

[Sanctuary]

Well that's disappointing ...

What I find frustrating is that it seems like the functionality is 'almost there', i.e. I can already click and drag to select sub-expressions, but when I press a key like SQRT, the whole sub-expression is then just replaced instead of edited. I am pretty sure it wouldn't be so hard to just take the selected piece and put it into the new expression (trivial for unary operators, but still could have a common sense default for things like fractions)

But thanks a lot for the code-snippet! That helps, especially for my example 2.