Keywords: ADT fold - sum.png en ADT fold sum LaTeXiT Daniel Brown LaTeX source \begin diagram 1 + \N \times \tt List _\N \rTo 1 + \N \times \N \\ \dTo \tt nil \tt cons \dTo_ 0 + \\ \tt List _\N \rDashto_ \tt sum \N \end diagram Linked lists Commutative squares |