Can't insert = between tokens in let-expression

[cdfa]

Hi! I was just playing around with the June 2022 version after the FoC meetup and thought I should report a bug I encountered.

First, I entered the expression let fact = x in a, then I remove the = and it becomes let factx in a. If I then insert a space between fact and x and move the cursor next to the x (so it becomes let fact |x in a with | as the cursor) and I try to insert a =, the = is not inserted and I get an exception:

(Exception
   "(Failure \"impossible: bin encountered empty or singleton stack\")")

I also just want to say that I love the concept and it looks gorgeous. I hope you manage to make this into a robust prototype!