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(a) The four-side polygon (shaded region) formed by four chords each of which connects two points on the two-turn reverse helical trajectory. The polygon plane is chosen to be parallel to y-z plane. (b) Continuous subsegment trajectories (thick curves) corresponding to sides i of the four-side polygon (dashed lines), and the directions of the chords (i.e., the sides) of the polygon, where i=1,2, and 3.
