[NTG-context] filling overlap ?

Could someone point me to te way as how to fill only the overlap of three paths using metafun ? path p ; p := fullsquare xscaled 10mm yscaled 12mm superellipsed .7 ; path q ; q := fullsquare xscaled 9mm yscaled 11mm superellipsed .7 ; q := q shifted (3mm, -2mm); path r ; r := fullsquare xscaled 8mm yscaled 10mm superellipsed .7 ; r := r shifted (0mm, -4mm); draw p ; draw q ; draw r ; Thanks .Floris