very difficult puzzle

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chesse_chames

Found this knights puzzle. Can not figure it out!

Eleven moves?!! seems impossible/impassable.

each one must move at least once, so that's eight moves. Then a few must move twice.

help with solving this, please.

http://www.ageofpuzzles.com/Puzzles/G-KnightsExchange/G-KnightsExchange.htm