9.5.1 Block Definitions

A block definition is, basically, a set containing zero or more other cells, including other sets. Refer to the section on sets for information on the type-specific parameters required. The one difference between a definition and a set is that a definition should not include any joint data. A block definition is only included in the image when it is referenced by an instance element. The definition itself is included within the first instance element that references it; block definitions cannot be included within a set and are not included in the member count of the set that contains the first instance. Block definitions can include references to other block definitions, but care must be taken to ensure that a referenced block does not refer back to the referencing block or an illegal circular relationship will result. It is recommended that block definitions be assigned a unique name, to avoid unexpected behavior during dereferencing.