void eval()
{

   TCanvas *c1 = new TCanvas("c1");
   c1->Range(0, 0, 20, 10);
   TPaveLabel *pt1 = new TPaveLabel(0.2, 4, 3, 6, "Eval");
   pt1->SetTextSize(0.5);
   pt1->SetFillColor(42);
   pt1->Draw();
   TPaveText *pt2 = new TPaveText(4.5, 4, 7.8, 6);
   pt2->Draw();
   TText *t1 = pt2->AddText("Read Operator");
   TText *t2 = pt2->AddText("number i");
   TPaveText *pt3 = new TPaveText(9, 3.5, 17.5, 6.5);
   TText *t4 = pt3->AddText("Apply Operator to current stack values");
   TText *t5 = pt3->AddText("Example: if operator +");
   TText *t6 = pt3->AddText("value[i] += value[i-1]");
   t4->SetTextAlign(22);
   t5->SetTextAlign(22);
   t6->SetTextAlign(22);
   t5->SetTextColor(4);
   t6->SetTextColor(2);
   pt3->Draw();
   TPaveLabel *pt4 = new TPaveLabel(4, 0.5, 12, 2.5, "return result = value[i]");
   pt4->Draw();
   TArrow *ar1 = new TArrow(6, 4, 6, 2.7, 0.02, "|>");
   ar1->Draw();
   TText *t7 = new TText(6.56, 2.7, "if i = number of stack elements");
   t7->SetTextSize(0.04);
   t7->Draw();
   ar1->DrawArrow(6, 8, 6, 6.2, 0.02, "|>");
   TLine *l1 = new TLine(12, 6.6, 12, 8);
   l1->Draw();
   l1->DrawLine(12, 8, 6, 8);
   ar1->DrawArrow(3, 5, 4.4, 5, 0.02, "|>");
   ar1->DrawArrow(7.8, 5, 8.9, 5, 0.02, "|>");
}