public override string CreateTreeRecursively(ref int nodeIndex, int prevIndex)
{
var graph = $"{Environment.NewLine}\tnode{nodeIndex} [ label = \"*\" ]";
if (prevIndex != 0)
{
graph += $"{Environment.NewLine}\tnode{prevIndex} -- node{nodeIndex}";
}
prevIndex = nodeIndex;
nodeIndex++;
graph += LeftOperand.CreateTreeRecursively(ref nodeIndex, prevIndex);
graph += RightOperand.CreateTreeRecursively(ref nodeIndex, prevIndex);
return(graph);
}