Sequence: send message to itself or polling

Often times, I need to send a message to itself or polling and in which case I need to do `A -> A: polling`, something like this:
```ascii
┌─┐
│A│
└┬┘
 │
 │Polling
 │-----+
 │     |
 │<----+
 │
┌┴┐
│A│
└─┘
```

However, it seems it does not work here: 
```bash
$ diagon Sequence -- "A -> A: polling"
Something went wrong!
free(): invalid next size (normal)
Aborted
```