+ void (*destroy) (struct spreadsheet *);
+ struct casereader* (*make_reader) (struct spreadsheet *,
+ const struct spreadsheet_read_options *);
+ const char * (*get_sheet_name) (struct spreadsheet *, int);
+ char * (*get_sheet_range) (struct spreadsheet *, int);
+ int (*get_sheet_n_sheets) (struct spreadsheet *);
+ unsigned int (*get_sheet_n_rows) (struct spreadsheet *, int);
+ unsigned int (*get_sheet_n_columns) (struct spreadsheet *, int);
+ char * (*get_sheet_cell) (struct spreadsheet *, int , int , int);