r->sheets = xrealloc (r->sheets, sizeof (*r->sheets) * ++r->n_allocated_sheets);
r->sheets[r->n_allocated_sheets - 1].start_col = -1;
r->sheets[r->n_allocated_sheets - 1].stop_col = -1;
r->sheets[r->n_allocated_sheets - 1].start_row = -1;
r->sheets[r->n_allocated_sheets - 1].stop_row = -1;
r->sheets = xrealloc (r->sheets, sizeof (*r->sheets) * ++r->n_allocated_sheets);
r->sheets[r->n_allocated_sheets - 1].start_col = -1;
r->sheets[r->n_allocated_sheets - 1].stop_col = -1;
r->sheets[r->n_allocated_sheets - 1].start_row = -1;
r->sheets[r->n_allocated_sheets - 1].stop_row = -1;