/// <param name="h">
/// Word index of governor (zero = root node; actual word
/// indexing begins at 1)
/// </param>
/// <param name="t">
/// Word index of dependent (zero = root node; actual word
/// indexing begins at 1)
/// </param>
/// <param name="l">Arc label</param>
public virtual void AddArc(int h, int t, string l)
{
tree.Set(t, h, l);
}
