idea is to have each of the various kinds of lines in the picture be emulated by some collection of localized structures in rule 110.

But at the outset it is by no means clear that collections of localized structures can be found that will behave in appropriate ways.

With some effort, however, it turns out to be possible to find the necessary constructs, and indeed the previous page shows various objects formed from localized structures in rule 110 that can be used to emulate most of the types of lines in picture (d) on page 679.

The first two pictures show objects that correspond to the black and white elements indicated by thick vertical lines in picture (d). Both of these objects happen to consist of the same four localized structures, but the objects are distinguished by the spacings between these structures.

The second two pictures on the previous page use the same idea of different spacings between localized structures to represent the black and gray lines shown coming in from the left in picture (d) on page 679.

Note that because of the particular form of rule 110, the objects in the second two pictures on the previous page move to the left rather than to the right. And indeed in setting up a correspondence with rule 110, it is convenient to left-right reverse all pictures of cyclic tag systems. But using the various objects from the previous page, together with a few others, it is then possible to set up a complete emulation of a cyclic tag system using rule 110.

The diagram on the facing page shows schematically how this can be done. Every line in the diagram corresponds to a single localized structure in rule 110, and although the whole diagram cannot be drawn completely to scale, the collisions between lines correctly show all the basic interactions that occur between structures.

The next several pages [684, 685, 686] then give details of what happens in each of the regions indicated by circles in the schematic diagram.

Region (a) shows a block separator—corresponding to a dashed line in picture (d) on page 679—hitting the single black element in the sequence that exists at the first step. Because the element hit is black, an object must be produced that allows information from the block at this step to pass through. Most of the activity in region (a) is concerned with producing such an object. But it turns out that as a side-effect two