How to convert range types 'a'..'z'?

[Hlaaftana]

The type of your first converter's argument is `(char, string)`, not `(string, 
string)` which is what you are trying to use. Also in your comment you say you 
tried `check[0]('a'..'z')` which isn't valid, the correct way would be 
`range['a'..'z'](check[0])`.