text: 4.8.2 Inverse Functional Properties 
  If a property is inverse functional then it means that the inverse property is functional. For a given 
  individual, there can be at most one individual related to that individual via the property. Following our 
  example from section 4.8.1 the inverse of hasBirthMother would be isBirthMotherOf. The 
  isBirthMotherOf property would not be functional since a woman can be the birth mother of several 
  children. However, it would be inverse functional since each person has exactly one mother. 
  
  4.8.3 Transitive Properties 