DEFINITION NO FURTHER A MYSTERY

There aren't any negligible pairs for tense/lax pairs for example /ʊ/ and /u/ in unstressed syllables, simply because they are in complementary distribution. They are not genuinely various phonemes.I do think the discussion about /x/ and /χ/ is superfluous. I checked several WIKI-web pages and whilst the a single claims that /x/ is Erroneous one