This does not work with :
labeloffset := 2mm;
path p, q, r ; pair z ; p := unitsquare scaled 4cm ; z := point 3.6 of p ; q := z--urcorner p ; r := q -- reverse bottomboundary p -- cycle ; draw p ; draw q ; draw r numberstriped (.25,10,5) withcolor blue ; draw thetextext.llft("B", llcorner p) ; draw thetextext.lrt ("C", lrcorner p) ; draw thetextext.ulft("A", ulcorner p) ; draw thetextext.urt ("D", urcorner p) ; draw thetextext.lft ("M", z) ; \stopMPpage