function t = postFix(x)
if nargin == 0
   x = {{5 2 '+'} {{{7 1 '*'} 6 '*'} {3 4 '*'} '+'} '+'};
end

if iscell(x)
   switch x{3}
      case '+'
         t = postFix(x{1}) + postFix(x{2});
      case '*'
         t = postFix(x{1}) * postFix(x{2});
   end
else
   t = x;
end

