SELECT "normalize"('abc', 'def');  -- run-time error
ERROR:  invalid normalization form: def
SELECT U&'\00E4\24D1c' IS NFC NORMALIZED AS test_nfc;
 test_nfc 
----------
 t
(1 row)

SELECT num, val,
    val IS NFC NORMALIZED AS NFC,
    val IS NFD NORMALIZED AS NFD,
    val IS NFKC NORMALIZED AS NFKC,
    val IS NFKD NORMALIZED AS NFKD
FROM
  (VALUES (1, U&'\00E4bc'),
          (2, U&'\0061\0308bc'),
          (3, U&'\00E4\24D1c'),
          (4, U&'\0061\0308\24D1c'),
          (5, '')) vals (num, val)
ORDER BY num;
 num | val | nfc | nfd | nfkc | nfkd 
-----+-----+-----+-----+------+------
   1 | äbc | t   | f   | t    | f
   2 | äbc | f   | t   | f    | t
   3 | äⓑc | t   | f   | f    | f
   4 | äⓑc | f   | t   | f    | f
   5 |     | t   | t   | t    | t
(5 rows)

SELECT is_normalized('abc', 'def');  -- run-time error
ERROR:  invalid normalization form: def