public override string ToPlainString() {
StringBuilder sb = new StringBuilder();
if(EquivalentNodes != null) {
for(int i = 0; i < EquivalentNodes.Count - 1; i++) {
sb.AppendFormat("{0}|", EquivalentNodes[i].ToPlainString());
}
sb.Append(EquivalentNodes.Last().ToPlainString());
}
return sb.ToString();
}