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nrqed

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The key step to define the kinematic associahedron is to impose ##X_{ij} \geq 0## and also the impose the ##c_{ij} > 0 ##. They do this without giving any motivation at all. I know that it gives the correct answer in the end, but I wonder if there is any way to motivate these choices, from a physics point of view (or short of that, from a mathematical point of view). It would be nice to get some intuition about what leads to these choices, other than "because it gives the correct final answer".

Thank you!