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[ddlm-group] A rationalisation of DDLm

Dear DDLm group,

In preparation for setting things in stone (famous last words) for Vol G second edition I have made some edits to the DDLm specification file ddl.dic to remove unused or obsolete tags. You can see the full changes in the pull request at https://github.com/COMCIFS/cif_core/pull/188 . In summary (from the pull request):

A number of items are obsolete in DDLm:

  1. Save frames. DDLm aims to be format-agnostic. All references to save frames and ref-loops have been removed, except for import statements for which a save frame reference is currently unavoidable
  2. LOOP category. No official dictionary (DDL1/2/m) has ever used more than 1 level of looping, so there is no need for such attributes, which are also CIF-format-specific.
  3. Complex data types. Data types of the form 'Type1|Type2' to indicate that an item could have more than one type are not used, are complex to implement, and so have been removed. Data types of the form List(Type2,Type3,...) to indicate List objects where each element is composed of a tuple of values, each of potentially different types, have been removed. These were almost exclusively used for synthetic keys, which should be opaque. Any other current use can be changed to Array type.
  4. Allow arrays of strings. The current DDLm dictionary restricts Arrays to numeric values for no apparent reason.

If these changes are considered significant enough, we should change the major version number to 4.0. Please comment on this.

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