batch.go
func (wb *WriteBatch) Delete(k []byte) error {
	wb.Lock()
	defer wb.Unlock()

	if err := wb.txn.Delete(k); err != ErrTxnTooBig {
		return err
	}
	if err := wb.commit(); err != nil {
		return err
	}
	if err := wb.txn.Delete(k); err != nil {
		wb.err = err
		return err
	}
	return nil
}
func (wb *WriteBatch) Delete(k []byte) error {
	wb.Lock()
	defer wb.Unlock()

	if err := wb.txn.Delete(k); err != ErrTxnTooBig {
		return err
	}
	if err := wb.commit(); err != nil {
		return err
	}
	if err := wb.txn.Delete(k); err != nil {
		wb.err = err
		return err
	}
	return nil
}
// Delete is equivalent of Txn.Delete.