Weston, Paul E.; Tuttle, H. (1967): Data structures for computations within networks of relations. B.C.L. Report No. 67.2, p. 35–37., p. 37.
A ring type of computer data structure was devised and given the name "Halo" to distinguish it from earlier ring structures, such as those of the CORAL data language, from which it differs significantly in structure. The scheme is shown in Fig. 3, in which each rectangle represents one computer word. There are two links in each word, and two distinguished "arcs" within the complete closed loop of the structure, designated by "s" and "ss" in the figure. As in ordinary list structures, each "link" consists of the address of some word in the computer memory, and this type of linkage forms the skeleton of the halo complex. At the two points of juncture, between the "s" and "ss" arcs there are distinguished words, the "head" and "Type node," which are marked by recognizable internal bit patterns. The address of the head is conveniently taken as the "name" of the Halo for programming purposes, but the head is not available for holding information not essential to marking the internal structure, while the type node is free to carry an extra symbol, which may be used as a "type" designation, or may name a data list containing fuller information. The linkages between halos are formed, as figure 3 shows, by the sharing of a common word. Because the s and ss links are distinguishable within words these inter‑halo linkages are inherently directed ones, a useful feature which had to be achieved by more complicated ad hoc means in earlier ring data structures.
The ring form of structure is of proven value in dealing with data having a complex internal structure, particularly when complicated classifications, or multiplace relations must be represented, and when maximum freedom is needed in choice of path in searching through stored data. A halo complex was programmed as a set of added primitives in the IBM 7094 IPL‑V system at Illinois, but IPL‑V has since become inoperative on the 7094 due to a change in staff at the digital computer laboratory. However a more sophisticated version of the halo complex has been recently developed and will be programmed for a Control Data 1604 in the near future.