Highlight lines
---------------

The following code snippet should dim all lines that are not marked with
``hl_lines``, and so the effect is to highlight the selected lines

.. code-block:: python
   :linenos:
   :hl_lines: 3 5 6

    def f():
        a = 1
        b = 2
        c = 3
        d = 4
        e = 5
        f = 6
        g = 7


