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Level 13 - UDIILBNG
The answer to this puzzle is:
Words: BIDING, IDLING, BLUING, LUBING, DUBLIN, BLINI, UNBID, LUNGI, BINDI, DING, GLIB, BUND, IBID, LING, BLIN, BIDI, BUNG, GILD, GUID, DUNG, NUB, LUG, DIN, BIN, GUN, GNU, GIN, BUN, DUN, LID, DUG, LIB, LIN, NIL, NIB
Bonus words:
BIDING, IDLING, BLUING, LUBING, DUBLIN, BLINI, UNBID, LUNGI, BINDI, DING, GLIB, BUND, IBID, LING, BLIN, BIDI, BUNG, GILD, GUID, DUNG, NUB, LUG, DIN, BIN, GUN, GNU, GIN, BUN, DUN, LID, DUG, LIB, LIN, NIL, NIB