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