Abbreviation for show-stack command?

I'm trying to create an abbreviation for the show-stack command, but it is unfortunately not woking:

```
  Pry.commands.alias_command "bt", "show-stack"
```

which yields:

```
1] pry(main)> bt
NameError: undefined local variable or method `bt' for main:Object
from (pry):1:in `get_scores'
```

when I try to use it. Any suggestions on how to accomplish this? TIA.
