tikz
draw[very thick,name path=A] (0,15) to [out=0,in=270] (15,30);
draw[very thick, name path=B] (0,15) to [out=0,in=90] (15,0);
draw[very thick,name path=C] (15,0) to [out=90,in=180] (30,15);
draw[very thick,name path=D] (30,15) to [out=180,in=270] (15,30);