A possible editing technique

First, this is a nice product with a very innovative interface. I like it, and I hope you will be able to continue to fix it and improve it.
Here's a modest suggestion for the editing interface.

Each empty box represents a leaf in the expression tree.
Consider that tapping on a leaf creates a selection box on that leaf and tapping on an explicit operator creates a selection box for the subexpression (although this doesn't work for implied multiplicaiton). Why not enable subsequent taps on the same item to simply enlarge the selection box to the next higher node in the expression tree?