-/* Within SOURCE, which must not have a backing casereader,
- writes the VALUE_CNT values in VALUES_CNT to the VALUE_CNT
- columns starting from START_COLUMN, in every row, even in rows
- not yet otherwise initialized. Returns true if successful,
- false if an I/O error occurs.
-
- We don't support backing != NULL because (1) it's harder and
- (2) this function is only called by
- datasheet_insert_column, which doesn't reuse columns from
- sources that are backed by casereaders. */