text: 3.1.2 Properties 
  Properties are binary relations between individuals. I.e., properties link two individuals together. For 
  example, the property hasFriend might link the individual Biswanath to the individual Michael, or 
  the property hasChild might link the individual Michael to the individual Oriana. Properties can have 
  inverses. For example, the inverse of hasChild is hasParent. Properties can be limited to having a 
  single value – i.e., to being functional. They can also be transitive or symmetric. These property 
  characteristics are explained in detail in Section 4.8. Figure 3.2 shows a representation of some properties. 