[NTG-context] About the macro path intersectionpoint path with Metapost

Hi, I would like to reproduce the attached figure. My problem is to create the points of intersection between the circles. It seems that my approach is not the right one. Thank you. Fabrice \starttext \startMPcode path p, q, r, b ; p:= fullcircle scaled 6cm ; q:= fullcircle scaled 4cm shifted (2.5cm,2cm) ; r:= fullcircle scaled 4cm shifted (-2.5cm,2cm); b:= fullcircle scaled 2cm shifted (0,-3cm); pair A, B ; A:= p intersectionpoint r ; B:= p intersectionpoint r ; path u ; u:= A--B--cycle ; draw p ; draw q ; draw r ; draw b ; draw u numberstriped (.25,15,5) withcolor magenta ; \stopMPcode \stoptext