a. Deletion and contraction in abstract graphs
b. Spanning trees
c. Planar duality: (Σ\e)* = Σ*/e*
d. Implementing deletion and contraction in maps
e. Cycle-cut duality
f. Tree-cotree decompositions (hint: cotrees are onions)
g. Euler's formula
h. The combinatorial Gauss-Bonnet theorem