================================================================================
gnuplot environment
================================================================================
foo
\begin{gnuplot}
bar
\end{gnuplot}
quux
--------------------------------------------------------------------------------
(document
  (text)
  (gnuplot_env
    (begin
      (cs)
      (group (l) (name) (r))
      (eol))
    (verbatim)
    (end (cs) (group (l) (name) (r))))
  (text))

================================================================================
gnuplot environment with optional parameter
================================================================================
foo
\begin{gnuplot}[wibble]
bar
\end{gnuplot}
quux
--------------------------------------------------------------------------------
(document
  (text)
  (gnuplot_env
    (begin
      (cs)
      (group (l) (name) (r))
      (brack_group (lbrack) (text) (rbrack))
      (eol))
    (verbatim)
    (end (cs) (group (l) (name) (r))))
  (text))