Dear Fabrice, Metapost is great for drawiang figures but it is not easy for me to use it.

b:= fullcircle scaled 2cm shifted (0,-3cm); pair A, B ; A:= p intersectionpoint r ; B:= p intersectionpoint r ; path u ; u:= A--B--cycle ;

A and B are the same point. One of “r” should be changed to “q” or “b”. Then you can have a line. I also made similar errors many times too. Btw, is there any reason to add “—cycle”? Enjoy the drawing. Best regards, Dalyoung