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 Th17:
  for x,H holds All(x,H).1 = 4
proof
  let x,H;
  thus All(x,H).1 = (<*4*>^(<*x*>^H)).1 by FINSEQ_1:32
    .= 4 by FINSEQ_1:41;
end;
