Give the sequence of trees produced by the horizontal-vertical line
intersection routine when run on the lines (2,1)---(2,5), (1,6)---(6,6),
(3,3)---(3,4), and (5,2)---(5,7).
Name the lines A, B, C, and D respectively.
Draw the depth-first search forest for the graph defined by the edges
AB AC AD AG DE EF FG FI FJ JI, assuming the adjacency matrix representation.