      \fitem{flatten}{(t:tab, l:numeric)} returns a new tab where \code{l}
      levels of nesting has been flattened. If \code{l == 0}, the function
      is the identity. If \code{l} is strictly negative, it is equivalent
      to \code{@flatten} without the level argument.
      \begin{lstlisting}
            @flatten([1, [2, [3, 3], 2], [[[4, 4, 4]]]], 0)
            ->[1, [2, [3, 3], 2], [[[4, 4, 4]]]]
            @flatten([1, [2, [3, 3], 2], [[[4, 4, 4]]]], 1)
            ->[1, 2, [3, 3], 2, [[4, 4, 4]]]
            @flatten([1, [2, [3, 3], 2], [[[4, 4, 4]]]], 2)
            ->[1, 2, 3, 3, 2, [4, 4, 4]]
            @flatten([1, [2, [3, 3], 2], [[[4, 4, 4]]]], 3)
            ->[1, 2, 3, 3, 2, 4, 4, 4]
            @flatten([1, [2, [3, 3], 2], [[[4, 4, 4]]]], 4)
            ->[1, 2, 3, 3, 2, 4, 4, 4]
            @flatten([1, [2, [3, 3], 2], [[[4, 4, 4]]]], -1)
            ->[1, 2, 3, 3, 2, 4, 4, 4]
      \end{lstlisting}



      %%% -------------------------------------------------------

