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;
