The accidental addition of a data-structure sentinel can cause
					serious programming logic problems.Data-structure sentinels are often used to mark the structure of data. A
						common example of this is the null character at the end of strings or a
						special sentinel to mark the end of a linked list. It is dangerous to allow
						this type of control data to be easily accessible. Therefore, it is
						important to protect from the addition or modification of sentinels.