reserve W for Universe,
  H for ZF-formula,
  x,y,z,X for set,
  k for Variable,
  f for Function of VAR,W,
  u,v for Element of W;

theorem Th2:
  W |= the_axiom_of_unions
proof
  for u holds union u in W;
  hence thesis by ZFMODEL1:4;
end;
