Read and write locks. The transaction manager of section 8.5 locks a cell upon its first use. If transactions T1 and T2 both want to read the same cell's content, then they cannot both lock the cell simultaneously. We can relax this behavior by introducing two kinds of locks, read locks and write locks. A transaction that holds a read lock is only allowed to read the cell's content, not change it. A transaction that holds a write lock can do all cell operations. A cell can either be locked with exactly one write lock or with any number of read locks. For this exercise, extend the transaction manager to use read and write locks.