ocr: 4.43. Now we understand that the cross-section of the band iS not an inter- val. [t must be a iolded interval with two layers, Let's follow the evolucion of thia colded interval as. il goes around, repeal- ing exacily the steps above, Starting another Lumn as a L-shaped channel, the stretch and fold voduccs a double-U- shape on relurn IO lle Poincare section.