   .. figure:: /_images/RPN_fig.jpg
      :width: 100%
      :align: center

   Hewlett-Packard made lots of calculators (left) using Reverse Polish Notation,
   which is a post-fix system for mathematical notion originally developed by
   `Jan_Łukasiewicz <https://en.wikipedia.org/wiki/Jan_Łukasiewicz>`_ (right).
   Here, operands are entered first followed by an operator, e.g., "3 5 +" instead
   of "3 + 5 =" (photo courtesy of John W. Robbins).
