sentences:  Cumulative hierarchy
  
  In mathematics, specifically set theory, a cumulative hierarchy is a family of sets "W"α indexed by ordinals α such that
  
  
  Some authors additionally require that "W"α+1 ⊆ "P"("W"α) or that "W"0 is empty.The union "W" of the sets of a cumulative hierarchy is often used as a model of set theory.