\dvips \usepackage[nohug]{diagrams} \diagram{ 1 + \mathbb N \times {\tt List}_{\mathbb N} & \rTo & 1 + \mathbb N \times \mathbb N \\ \dTo^{[{\tt nil}, {\tt cons}]} & & \dTo_{[0,+]} \\ {\tt List}_{\mathbb N} & \rDashto_{\tt sum} & \mathbb N }