    |          |  \ /   \
    |    Fact  | &lt;-¤-D   o Y
    |          |  / \   /
    |          | I   &gt; /
    |          |      /
    |          |     /
    |          |    /  Case
    |          |   /
    |          |  /
    |          | /
    |          |/
    |          o
    |          X
    |
    | Figure 1.  Basic Structure &amp; Terminology

JA: We usually think of each duction as being produced from a vertext
    to its opposite side, thinking of the vertex as an index of its
    two incident