theorem
  M |= H => Ex(x,H)
proof
  let v;
  M,v/(x,v.x) |= H implies M,v |= Ex(x,H) by Th73;
  then M,v |= H implies M,v |= Ex(x,H) by FUNCT_7:35;
  hence thesis by ZF_MODEL:18;
end;
