Chapter 9: Fundamental Physics

Section 10: The Sequencing of Events in the Universe

The sequential limit [in generalized substitution systems]

Even when the order of applying rules does not matter, using the scheme of a sequential substitution system will often give different results. If there is a tree of possible replacements (as in "A"->"AA"), then the sequential substitution system in a sense does depth-first recursion in the infinite tree, never returning from the single path it takes. Other schemes are closer to breadth-first recursion.

From Stephen Wolfram: A New Kind of Science [citation]