code: function() {
                  var A = this.start.elements, B = this.end.elements;
                  var C1 = B[0] - A[0], C2 = B[1] - A[1], C3 = B[2] - A[2];
                  return Math.sqrt(C1*C1 + C2*C2 + C3*C3);
                }
docs: Returns the length of the line segment