Ropes for Swift based on the article by Boehm, Atkinson, and Plass.
zone: a rope may contain zero or more nested spans called zones. Sometimes, in my notes and code comments, I use parentheses to denote zones.
Note: zones used to be "extents".
zone controller: every zone has an associated controller that either modifies and performs or suppresses rope insertions, deletions, et cetera, that overlap the zone.
boundary: the distance from the interior of a zone to its exterior, or vice versa, is one boundary
step: the distance of one boundary or across one UTF-16 code unit is one step
unit: the distance across one UTF-16 code unit is one unit
jot: the distance of one step or across one index is a jot
dimensions: a bundle of a subrope's measurements in units, jots, steps, and boundaries are its dimensions
axis: an axis names one measurement in a subrope's dimensions; I may rename "axis" to "scale".
index: a labeled entity embedded in a rope, or a struct Rope.Index
identifying such an entity by its label.
adjacent: two indices are adjacent if no steps separate them
alias: two indices alias one another if they are adjacent
label: an object that uniquely identifies an index
expired: each embedded index holds a weak reference to its label; an
index whose reference turns to nil is expired. Expired indices
are cleaned up by occasional garbage-collecting sweeps.