public void SetExpansionSegment()
{
expansion_StartSpan = null;
expansion_EndSpan = null;
foreach (Span s in startSpans)
{
if (!endSpans.Contains(s))
{
expansion_StartSpan = s;
break;
}
}
foreach (Span s in endSpans)
{
if (!startSpans.Contains(s))
{
expansion_EndSpan = s;
break;
}
}
if (expansion_EndSpan != null)
expansion_StartSpan = null;
}