The computer, under program control, would issue detailed instructions to the arm. If it could build any structure it could easily build a copy of itself, and hence was self-replicating. Self-replicating systems need not inherently be vastly complex. Simple existing biological systems, such as bacteria, have a complexity of about 10 million bits. Of course, a significant part of this complexity is devoted to mechanisms for synthesizing all the chemicals needed to build bacteria from any one of several simple sugars and a few inorganic salts, and other mechanisms for detecting and moving to nutrients.
Bacteria are more complex than strictly necessary simply to self-reproduce.
This would make it almost 10 thousand to 100 thousand times more complex than a bacterium, and a million times more complex than von Neumann's theoretical proposal. This level of complexity puts the project near the limits of current capabilities. The SRS has to exist in a complex lunar environment without any human support. The complexity estimate for the orbital site map alone is 100 billion bits, and the facilities for mining and refining the lunar soil have to deal with the entire range of circumstances that arise in such operations. When we contrast this with a bacterium, much of the additional complexity is relatively easy to explain.