On Feb 9, 2015, at 1:46 PM, Fabrice <couvreur.fabrice@wanadoo.fr <mailto:couvreur.fabrice@wanadoo.fr>> wrote:
> 
> Great, but I did not know the macro numberstriped. Is it a Metapost or 
> Metafun macro?



It is a MetaFun macro in mp-tool, afaik.

The idea of filling a path is sometimes clipping to a path. Here is a way to do your task with (almost) only MetaPost:

\startMPpage[offset=2mm]
 path p, q ;
 p := unitsquare scaled 4cm ;
 z1 = point 3.6 of p ;
 q := z1 -- urcorner p -- lrcorner p -- llcorner p -- cycle;
 for i = 8 step -.5 until 0:
   draw (i,0)*cm -- (i-4,4)*cm withpen pencircle scaled 2pt withcolor blue;
 endfor;
 clip currentpicture to q;
 draw p ; draw q ;
 label.llft("B", llcorner p) ;
 label.lrt ("C", lrcorner p) ;
 label.ulft("A", ulcorner p) ;
 label.urt ("D", urcorner p) ;
 label.lft ("M", z1) ;
\stopMPpage

…just in case it helps to clarify.

John