In what is one of the most toilsome exercises of the quarters, I reminded (hopefully!) the students of their graph terminology. Rigorous definitions for graphs, connected, cycle, acyclic, tree, subgraph, spanning tree, weighted graph, weight, minimum spanning tree.
Then we started to prove Kruskal's algorithm correct. This became quite rushed, and I was unhappy for having done it too "abstractly", so I decided I would redo it at the beginning of the next lecture in a more concrete way.