For sake of the golf problem, your example would return all 3 cases if m,n is 2,3. Extra credit for not including and/or removing phrases of x words that are part of phrases with y (y > x) words in the returned list.

And the list must be of non-null words, so no trivial solutions :-)

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Dr. Michael K. Neylon - mneylon-pm@masemware.com || "You've left the lens cap of your mind on again, Pinky" - The Brain