Take 2 strings s1 and s2 including only letters from a to z.

Checking all the time for each 'kata' is not the point of the katas (I think).

If you do, you will spend half the time on the site checking for proper inputs.

After reviewing the task description, I understand your concerns, but please note that the first line of the task clearly states: "Take 2 strings s1 and s2 including only letters from a to z". Therefore, we can assume that the input will only contain lowercase letters from 'a' to 'z' and we do not need to check for other characters.