      \fitem{map_concat}(a:map, b:map): returns a map \code{c} which
      contains the (key, value) pairs of \code{a} and a pair $(n, e)$ for
      each pair $(k, v)$ in \code{b} with $n$ ranging from \code{@size(a)}
      to  \code{@size(a) + @size(b)}. If \code{a} and \code{b} are
      ``vectors'' (\emph{i.e.} the range is an interval $[0, p]$ for some
      $p$), then \code{@map_compose} is the vector concatenation. 



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

