public override string CreateGraph(ref int index, int preIndex)
{
string graph = String.Format(Environment.NewLine + "node{0} [ label = \"\u2227\" ]", index);
int pre = index;
if (preIndex != 0)
{
graph += String.Format(Environment.NewLine + "node{0} -- node{1}", preIndex, index);
}
index++;
graph += LeftOperand.CreateGraph(ref index, pre);
graph += RightOperand.CreateGraph(ref index, pre);
return(graph);
}