title: Algorithm to Find Overlapping Line Segments
body: <pre>
  n4------------------n3--------------------n2--n1
  |                    |                    |    |
  |                    |                    | P1 |
  |                    |                    |    |
  |                    |                    n6--n5
  |                    |                    |
  |              n11--n10                   |
  n17      P4     |    |         P2         |
  |               | P3 |                    n7
  |              n12---n9                   |
  |               |                         n8
  |               |                         |
  n16------------n15---------n14------------n13
  </pre>
  
  <p>In the above ASCII art, there are four polygons (P1, P2, P3, P4) with exactly-overlapping line segments. For example, polygon P2 (formed by line segments between nodes n3, 10, 9, 12, 15, 14, 13, 8, 7, 6, and 2) and P1 (n1, 2, 5, and 6) overlap at the line segment between n2 and n6.</p>
  
  <p>What is the fastest way to find line segments that overlap exactly?</p>