reserve x for object;
reserve D for set;
reserve p for PartialPredicate of D;
reserve D for non empty set;
reserve p,q,r for PartialPredicate of D;

theorem Th15:
  PP_or(PP_or(p,q),PP_or(p,r)) = PP_or(PP_or(p,q),r)
  proof
    PP_or(p,PP_or(p,q)) = PP_or(PP_or(p,p),q) by Th14;
    hence thesis by Th14;
  end;
