f (l:ls) | "\\begin{tabbing}" `isPrefixOf` l = 
            f $ onHead (drop (length "\\end{tabbing}")) $ dropWhile (not . ("\\end{tabbing}" `isPrefixOf`)) ls
         | otherwise = l:f ls
f (l:ls) | "\\begin{tabbing}" `isPrefixOf` l = 
            f $ onHead (drop (length "\\end{tabbing}")) $ dropWhile (not . ("\\end{tabbing}" `isPrefixOf`)) ls
         | otherwise = l:f ls
f (l:ls) | "\\begin{tabbing}" `isPrefixOf` l = 
            f $ onHead (drop (length "\\end{tabbing}")) $ dropWhile (not . ("\\end{tabbing}" `isPrefixOf`)) ls
         | otherwise = l:f ls