Fig. 1.

Recursive and non-recursive procedures to generate hierarchies. (A) Simple iterative procedures add elements to a hierarchy, within fixed levels. In order to generate a level under ‘C’, another rule would be required – add another ‘D’ under each ‘C’. (B) Recursive rules are more abstract. They can be used to characterize hierarchical relations across many different levels of the hierarchy. With the same rule (B), an infinite number of hierarchical levels could be added to the structure.