Fix ruby version.
Use attr_reader.
Remove unused if condition.
Rename arg.
Introduce normalized_input method.
Remove attr_reader.
Refactoring if condition.
Return empty string when '0' not exists.
Combine lines.
Rename arg.
Simplify specification.
Refactoring if condition.
Extract method.
Use guard condition in method.
Remove local variable.
Remove local variable.
Remove unused first element in letters.
Refactoring.
Remove guard condition.
Remove useless specs.