“…(In Figure 11, the rungs [45, 46], [27,28], and [13,14] form these gates.) Every channel is paired with a neighboring "dead end" (ladder): in the figure, channel I is paired with the dead end extending between [43, 44] and [1,2], channel II is paired with the dead end between [29,30] and [3,4], and III is paired with the dead end between [11,12] and [5,6]. For shorthand, we define A to be channel I and its paired dead end, B to be II and its dead end, and C to be III and its dead end.…”