They borrow aspects of navigational databases, hierarchical databases and relational databases. "Namespaces" are a mechanism for logically grouping similar classes into a hierarchical structure. Complete-linkage clustering is one of several methods of agglomerative hierarchical clustering. The classification consists of five levels of hierarchical structure. The universe can be viewed as having a hierarchical structure. The American Mafia operates on a strict hierarchical structure. Contigs therefore provide the framework for hierarchical sequencing. Excitation and inhibition of behavior act at various levels of this hierarchical structure. The iterative construction leading to a hierarchical network. Ruskin believed in a hierarchical social structure. Several methods have been proposed based on hierarchical classification. Some databases provide specialised syntax for hierarchical data. An alternative approach is to simulate capabilities using commonly supported hierarchical domains. Complex member variables for contracts including arbitrarily hierarchical mappings and structs were supported. Social constructionists consider gender roles to be hierarchical and patriarchal.