reserve x,y,z,X,Y for set;
reserve X,Y for non empty set,
  f for Function of X,Y;
reserve X, Y for non empty set,
  F for (BinOp of Y),
  B for (Element of Fin X),
  f for Function of X,Y;
reserve A for set,
  x,y,z for Element of Fin A;

theorem Th37:
  {}.A is_a_unity_wrt FinUnion A
proof
  thus for x holds FinUnion A.({}.A, x) =x
  proof
    let x;
    thus FinUnion A.({}.A, x) = {} \/ x by Def4
      .= x;
  end;
  let x;
  thus FinUnion A.(x, {}.A) = x \/ {} by Def4
    .= x;
end;
