Section 3.1.9.1. A dictionary merging protocol

The following protocol describes the construction of a composite, or virtual, dictionary by merging and overlaying fragments of a local validation dictionary and the public dictionaries referenced from within a data file. The term `dictionary fragment' refers here to a physical disk file which contains one or more data blocks or save frames (according to whether the relevant data model is DDL1 or DDL2) containing complete or partial sets of attributes associated with data names identified in the relevant dictionary data block or save frame through the item _name (DDL1) or _item.name (DDL2).

(i) Assemble and load all dictionary fragments against which the current data block will be validated. The order of presentation is important. Complete dictionaries referenced by a data file should be assembled in the order cited. A dictionary validation application may then accept a list of additional dictionary fragments to PREPEND to, REPLACE or APPEND to each file in the list of cited dictionaries. In most applications, it will be appropriate to append to or replace attributes defined in a public dictionary, and the PREPEND operation is presented only for completeness.

(ii) Define three modes in which conflicting data names in the aggregate dictionary file may be resolved, called STRICT, REPLACE and OVERLAY.

(iii) Scan the aggregate dictionary fragments in the order of loading. Assemble for each defined data name a composite definition frame (data block or save frame as appropriate) as follows, depending on the mode in which the validation application is operating:

STRICT: If a data name appears to be multiply defined, generate a fatal error. This mode permits the interpolation of local dictionaries that do not attempt to modify the attributes of public data items.

REPLACE: All attributes previously stored for the conflicting data name are deleted, and only the attributes in the later data block (or save frame) containing the definition are preserved. This mode permits the complete redefinition of public data names and is not appropriate for validation of CIFs to be archived. Its main use would be in testing modifications of individual definition frames outside the parent dictionary.

OVERLAY: New attributes are added to those already stored for the data name; conflicting attributes replace those already stored. This is the standard mechanism for modifying attributes for application-specific validation purposes.

This protocol allows the creation of a coherent virtual dictionary from several different dictionary files or fragments. Although it must be used with care, it permits different levels of validation based on dictionary-driven methods without modifying the original dictionary files themselves.

As an example, consider the core CIF dictionary definition mentioned above of the number of hydrogen atoms that might be attached to an atom site (Example 3.1.9.1).

For a particular application, any structures reporting more than four attached hydrogen atoms might be considered as invalid. A validation program to satisfy this requirement might therefore build a composite dictionary from the public cif_core.dic, which contains the definition in Example 3.1.9.1, and the fragment of Example 3.1.9.2, processed in APPEND/OVERLAY modes.