-/* There is a lot of potential confusion in the vfm and related
- routines over the number of `value's at each stage of the process.
- Here is each nval count, with explanation, as set up by
- open_active_file():
-
- temp_dict->nval: Number of `value's in the cases after the
- transformations leading up to TEMPORARY have been performed.
-
- compaction_nval: Number of `value's in the cases after the
- transformations leading up to TEMPORARY have been performed
- and the case has been compacted by compact_case(), if
- compaction is necessary. This the number of `value's in the
- cases saved by the sink stream. (However, note that the cases
- passed to the sink stream have not yet been compacted. It is
- the responsibility of the data sink to call compact_case().)
- `compaction' becomes the new value of default_dict.nval after
- the procedure is completed.
-
- default_dict.nval: This is often an alias for temp_dict->nval.
- As such it can really have no separate existence until the
- procedure is complete. For this reason it should *not* be
- referenced inside the execution of a procedure. */
-/* Makes all preparations for reading from the data source and writing
- to the data sink. */