reserve k,m,n for Element of NAT,
  a,X,Y for set,
  D,D1,D2 for non empty set;
reserve p,q for FinSequence of NAT;
reserve x,y,z,t for Variable;
reserve F,F1,G,G1,H,H1 for ZF-formula;

theorem
  All(x,y,H) = All(x,All(y,H)) & Ex(x,y,H) = Ex(x,Ex(y,H));
