Also known as 2421 code
complementary binary-coded decimal which assigns a group of 4 bits to each decimal digit: 0→0000, 1→0001, 2→0010, 3→0011, 4→0100, 5→1011, 6→1100, 7→1101, 8→1110, 9→1111
Discovered by embedding cosine similarity (sentence-transformers MiniLM, 384-dim).