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Propagating the Overlap
The first (blue) point (index 0) of the measure controlled by directrices D_{1} and D_{2} is here shown being overlapped
by the n'th (blueturnedconvenientlyred) point (index n1) of the same ^{(*)} cycle,
which completes that cycle, and
starts the next.
So point n1 of any cycle is point 0 of the following cycle.
Accordingly, this measure has a finite steppingrate of n steps per cycle (s.c^{1}).

Among other elements, you may drag point H to adjust the overlap. Note that when the overlap is degenerate (when the last point of the cycle is incident with the first), the cycling is exact, and the measure is stationary. When the overlap is an interval (when the last point of the cycle is not incident with the first, and in fact steps "beyond" it), the cycling is inexact, and the measure seems to shift, or “precess”. 

We project, and so “measure”, the overlap
This allows us, using F and D_{1}, to “attach” the geometrically constant overlap (the black bars) 
In this fashion the overlap is propagated through cycles.
—for, after n steps, when a new cycle commences,
the overlaps are themselves identically overlapped.
If, during a pass, “old” points were replaced by “new”, somewhat as frames of a “movie” are replaced,
each of the old points would seem to move to its new position,
and the entire measure would appear to shift uniformly along.
^{*} Caveat: There is no actual guarantee that just these red positions depicted here, and said to be overtaking or overlapping the blue, are in fact the continuation round the conic of the points of the same, originallyblue, cycle. They could be the points of another distribution, one also controlled by the directrices D1 and D2. The overlap shown is a necessary, but not sufficient condition for such continuity. For our present purpose  namely, illustration  we assume the continuity.
The difficulty just noted is fundamental →.