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Donkey Kong Plus was a tech demo or game for Game Boy Advance, with Nintendo GameCube support. It was featured at E3 2002.
Summary
Donkey Kong Plus was intended as a successor to Donkey Kong on the Game Boy, which would feature the addition of a level editor and gameplay similar to the original game. Plans for the game would change, and this game would materialise as Mario vs. Donkey Kong, including leftover unused code for a level editor.
References