Hunt the boundaries

countUniqueWords passes every happy-path test. Write main printing one PASS/FAIL line per edge case you believe it must survive: empty string, only whitespace, single word, duplicated words ("the the"), mixed case ("The THE"), and words with punctuation adjacent ("hello,world"). Run it - at least one case should surprise you - then harden countUniqueWords so every one of your cases prints PASS (treat "hello,world" as two words by splitting on any non-letter).

Difficulty: advanced