      \fitem{history_length}(\emph{variable}) : This is a special
      form. It returns the maximal length of the history of a
      variable, \emph{i.e.} the number of update that are recorded. 
      See. sect.~\ref{sec:mapvariable} page~\pageref{sec:mapvariable}
      and the  \code{@map_history_XXX} \code{@tab_history_XXX}
      functions.  



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

