The figure shows a (convex) polygon with nine vertices. The six diagonals which have been drawn dissect the polygon into the seven triangles: P0P1P3, P0P3P6, P0P6P7, P0P7P8, P1P2P3, P3P4P6, P4P5P6. In how many ways can these triangles be labeled with the names △1, △2, △3, △4, △5, △6, △7 so that Pi is a vertex of triangle △i for i=1,2,3,4,5,6,7? Justify your answer.
6740