prologues := 3;beginfig(1); u = 1cm; z0 = (0,6u); z1 = (6u,-3u); z2 = (-4u,-2u); z3 = (-6u,2u); draw z0--z1--z2--z3--cycle; draw z0--z2; draw z1--z3; path da,db; da = ((1.1z1-0.1z3)--(0.8z3+0.2z1)) shifted 0.5(z0+z2-z1-z3); db = ((1.1z0-0.1z2)--(0.9z2+0.1z0)) shifted 0.5(z1+z3-z2-z0); draw da; draw db; dotlabel.top(btex $A$ etex, z0); dotlabel.rt(btex $B$ etex, z1); dotlabel.bot(btex $C$ etex, z2); dotlabel.lft(btex $D$ etex, z3); dotlabel.ulft(btex $O$ etex, (da intersectionpoint db)); dotlabel.ulft(btex $J$ etex, (db intersectionpoint (z1--z3))); dotlabel.urt(btex $M$ etex, 0.5[z0,z1]); dotlabel.lrt(btex $N$ etex, 0.5[z1,z2]); dotlabel.llft(btex $P$ etex, 0.5[z2,z3]); dotlabel.ulft(btex $Q$ etex, 0.5[z3,z0]); draw (db intersectionpoint (z1--z3))--0.5[z0,z1]--0.5[z1,z2]--cycle withcolor red; pickup pencircle scaled 1bp ; draw (da intersectionpoint db)--0.5[z0,z1]--z1--0.5[z1,z2]--cycle; endfig; end