diff -u -r1.2 -r1.3
--- TrackToTrackDOCA.java	29 Mar 2007 19:12:12 -0000	1.2
+++ TrackToTrackDOCA.java	2 Jan 2008 23:20:39 -0000	1.3
@@ -21,7 +21,7 @@

   * four hits (otherwise the direction is not so meaningful).
   *
   * @author Mat Charles <[log in to unmask]>

-  * @version $Id: TrackToTrackDOCA.java,v 1.2 2007/03/29 19:12:12 mcharles Exp $

+  * @version $Id: TrackToTrackDOCA.java,v 1.3 2008/01/02 23:20:39 mcharles Exp $

   */
 
 public class TrackToTrackDOCA implements StructuralLikelihoodQuantity

@@ -48,7 +48,18 @@

 	Line line2 = MiscUtilities.makeLine(track2);
 
 	double[] distancesAlongLinesToPOCAs = Line.getPOCAOfLines(line1, line2);

+
+	// The POCA-finding code breaks down if the lines are parallel. Check for that:
+	boolean linesAreParallel = false;

 	if (distancesAlongLinesToPOCAs == null) {

+	    // They are exactly parallel
+	    linesAreParallel = true;
+	} else if (Double.isInfinite(distancesAlongLinesToPOCAs[0]) || Double.isInfinite(distancesAlongLinesToPOCAs[1])) {
+	    // Another symptom of parallel lines.
+	    linesAreParallel = true;
+	}
+
+	if (linesAreParallel) {

 	    // Lines are parallel!
 	    // Find a point on each line
 	    SpacePoint pointOnLine1 = line1.getPointAtDistance(0.0);