Runtime error when re-inserting then-token

[cdfa]

A runtime error I encountered while playing around in the june version:

1. enter a conditional like if x then y else z
2. remove then, so it becomes if x|y else z
3. enter a space and move left to make if x| y else z
4. attempt to enter then
5. observe runtime error:

Async_kernel: Unhandled exception (Failure "impossible: bin encountered empty or singleton stack")

And maybe this is a separate bug, but if you go
4. enter then (without a leading space) to make if x|then y else z
5. separate x from then with a space to make if x |then y else z
6. observe that the then is not recognized as a keyword and remains in the backpack