GetPossibleStrings() is an IEnumerable, but under the surface it returns a list.
It's somewhat a combination of two different worlds.
The main function Possibilities() has to call ToList() because of that.
Two routes that might have been a bit cleaner:

1. Let GetPossibleStrings() return List directly,
   since that's already what is built in the method body and what is required
   as the return value.

2. Make GetPossibleString() a real IEnumerable generator function, by
   making use of "yield return ...". This prevents the creation of intermediate
   lists when recursing. I followed this route in my own code, so if you want
   an example of this, take a look there.

Kind regards,
Maurice