“…However, examples of universal computers have been far less common, though diverse, based for example on silicon chips, DNA, neural nets, collective behavior, molecular arrays, quantum mechanics, collision systems and specialized fluid-flow geometries (e.g. Siegelmann and Sontag, 1995; Solé and Delgado, 1996; Nielsen and Chuang, 1997; Adamatzky, 2002; Benenson and others, 2004; De Silva and others, 2006; Prakash and Gershenfeld, 2007). Because of the search for practical devices, most known universal systems are microscopic, and in most cases the construction of general-purpose computation is by design and is rarely an accidental by-product.…”