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Conversion of writhe to internal twist; during this deformation, 
decreases continuously from 1 to 0, and 
increases continuously from 0 to 1; the central curve passes at some instant through an inflectional configuration, and at this instant 
decreases by unity and 
increases from 0 to 1. [Reproduced with permission from ref. 20 (Copyright 1992, The Royal Society).]
