NULL is the lack of any string.
If one view a string as a result of an operation, then an INVALID string is the consequence of bad input to an operation.
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.
strstr(NULL, /* valid string */)
strstr(/* valid string */, NULL)