The <importBase> element allows for incorporating an entity base (represented by <connectorBase>, <regionBase>, <descriptorBase>, <ruleBase> or <transitionBase> elementsdefined in an external document) to another already-defined base. ItsdocumentURIattribute refers to a URI corresponding to the NCL document containing the base to be imported. The alias attribute specifies a name to be used as prefix when referring to elements of this imported base. The reference must have the format: alias#element_id.

The import operation is transitive, that is, if baseA imports baseB that imports baseC, then baseA imports baseC. However, the alias defined for baseC inside baseB is not considered by baseA.

The following constraints must be satisfied:

• the <descriptorBase> element may have a child <importBase> element referring to a URI corresponding to another NCL document containing the descriptor base to be imported and nested. When a descriptor base is imported, the region base, the transition base and the rule base, when present in the imported document, are also automatically imported to the corresponding region and rule bases of the importing document;
• the <connectorBase> element may have a child <importBase> element referring to a URI corresponding to another connector base to be imported and nested;
• the <transitionBase> element may have a child <importBase> element referring to a URI corresponding to another transition base to be imported and nested;
• the <ruleBase> element may have a child <importBase> element referring to a URI corresponding to another NCL document containing the rule base to be imported and nested;
• the <regionBase> element may have a child <importBase> element referring to a URI corresponding to another NCL document containing the region base to be imported and nested. As the referred document URI can have more than one region base, the base to be imported must be identified by assigning its idvalue to the baseId attribute of the <importBase> element. On importing a <regionBase>, an attribute named region may be specified within the <importBase> element. When present, this attribute identifies the id of a <region> element declared in a <regionBase> element of the host document (the document that does the importing operation). As a consequence, all child <region> elements of the imported <regionBase> shall be considered as child <region> elements of the region referred by the <importBase>’s region attribute. If not specified, the child <region> elements of the imported <regionBase> shall be considered direct children of the host document <regionBase> element.