size(200); real w=1.35; path[] p; for(int k=0; k < 2; ++k) { int i=2+2*k; int ii=i^2; p[k]=(w/ii,1){1,-ii}::(w/i,1/i)::(w,1/ii){ii,-1}; } path q0=(0,0)--(w,0.5); path q1=(0,0)--(w,1.5); draw(q0); draw(p[0]); draw(q1); draw(p[1]); path s=buildcycle(q0,p[0],q1,p[1]); fill(s,mediumgrey); label("$P$",intersectionpoint(p[0],q0),N); label("$Q$",intersectionpoint(p[0],q1),E); label("$R$",intersectionpoint(p[1],q1),W); label("$S$",intersectionpoint(p[1],q0),S); label("$f > 0$",0.5*(min(s)+max(s)),UnFill);