public RelNode(FactorNode lf, string op, FactorNode rf) : base(NodeType.RelNode)
{
l_factor = lf;
r_factor = rf;
Op = op;
children = new List <BaseNode>();
children.Add(l_factor);
children.Add(r_factor);
}
public RelNode(FactorNode factor) : this(factor, null, null)
{
}