Defining DeptDirectory Data
¥A DeptDirectory is either:
ÐEmpty(), the empty DeptDirectory , or
ÐNonEmpty(first,rest), a non-empty DeptDirectory, where first is an Entry and rest is a DeptDirectory

¥Examples:
¥ Empty()
NonEmpty(Entry("Stephen","DH 3103","x 3846"),
         Empty())
 
Ð