diff -u -r1.3 -r1.4
--- TrackToTrackDOCA.java	2 Jan 2008 23:20:39 -0000	1.3
+++ TrackToTrackDOCA.java	9 Jan 2008 18:44:03 -0000	1.4
@@ -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 1.3 2008/01/02 23:20:39 mcharles Exp $

+  * @version $Id: TrackToTrackDOCA.java,v 1.4 2008/01/09 18:44:03 mcharles Exp $

   */
 
 public class TrackToTrackDOCA implements StructuralLikelihoodQuantity

@@ -58,7 +58,8 @@

 	    // Another symptom of parallel lines.
 	    linesAreParallel = true;
 	}

-

+	
+	double doca = -1.0;

 	if (linesAreParallel) {
 	    // Lines are parallel!
 	    // Find a point on each line

@@ -74,14 +75,22 @@

 	    double direction_z = line1.getDirection().z();
 	    double dotProduct = displacement_x*direction_x + displacement_y*direction_y + displacement_z*direction_z;
 	    // Subtract in quadrature to get perpendicular distance (DOCA)

-	    double doca = distanceBetweenPoints*distanceBetweenPoints - dotProduct*dotProduct;
-	    return doca;

+	    doca = distanceBetweenPoints*distanceBetweenPoints - dotProduct*dotProduct;

 	} else {
 	    Hep3Vector poca1 = line1.getPointAtDistance(distancesAlongLinesToPOCAs[0]);
 	    Hep3Vector poca2 = line2.getPointAtDistance(distancesAlongLinesToPOCAs[1]);

-	    double doca = VecOp.sub(poca1,poca2).magnitude();
-	    return doca;

+	    doca = VecOp.sub(poca1,poca2).magnitude();
+	}
+	
+	// DOCA should never be less than zero
+	if (doca < 0.0 && doca > -1.0E-10) {
+	    // Rounding error -- ignore it
+	    doca = 0.0;
+	} else if (doca < 0.0) {
+	    // Error
+	    throw new AssertionError("DOCA is "+doca+" but values below zero are forbidden! Flag [linesAreParallel]="+linesAreParallel);

 	}

+	return doca;

     }