If you have studied Computer Science, you should know that the null string is quite a valid string.

Let's take strstr, which finds a matching substring needle in a haystack string.

-returns a NULL string if the needle is not in the haystack. -returns pointer to first matching substring.

Extend strstr with VALIDITY

Understood behaviour if both are valid.

Say the haystack is INVALID...as the return value is NULL or a strict substring of haystack, should return INVALID. A poison haystack should poison dependent strings.

Say the haystack is valid but the needle is INVALID...should return NULL. A valid string never contains an INVALID string as a subsequence.

Here's my behavior:

  strstr(NULL, /* valid string */)

I can't find the needle in the haystack (actually, I can't find anything in the haystack. I can't find the haystack.) Thus I return NULL.

  strstr(/* valid string */, NULL)

I can't find the needle in the haystack (actually, I wouldn't be able to find it: I don't know what I'm looking for.) Return NULL.

You're explanation is not inconsistent with my proposal, but you don't seem to grasp VALIDITY.