beginfig(1) a=.7in; b=0.5in; z0=(0,0); z1=(a,0); z2=(0,b); z0=.5[z1,z3]=.5[z2,z4]; draw z1..z2..z3..z4..cycle; drawarrow z0..z1; drawarrow z0..z2; label.top(btex $a$ etex, .5[z0, z1]); label.lft(btex $b$ etex, .5[z0, z2]); endfig end