Features in Binary Predecessor Tree

-- "Left descents" are headed by an extension and terminated by a "leaf node".
     (These will later be called "left descent assemblies" (l.d.a.s) because they recur as units.)

-- "Leaf nodes" have no predecessor (i.e. left child).
-- All nodes have extensions (i.e. infinite "right descents").

-- Integers in left descents wander up and down in magnitude, producing the "hailstone" effect.

-- A parent in {1[3]} uses i=2 and produces a bigger predecessor (b-step).
-- A parent in {2[3]} uses i=1 and produces a smaller predecessor (s-step).