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