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STEP 5
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Assign gradient to t[2] and t[-2], considering the direction of AA[2] and AA[-2] respectively.
  
Gradient of t[2]
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As gradient, t[2] could assume either x1x3x4 (left) or x2x3x4 (right). And t[2] is closer to AA[2] when t[1] assumes x1x3x4.