// Mesh

border a(t=0, 1) {x=t; y=0;, z=0; label=1;};

border b(t=0, 2) {x=1; y=t;, z= label=1;};

border c(t=1, 3) {x=t; y=2; label=1;};

border d(t=0, 2) {x=3; y=2-t; label=1;};

border e(t=0, 1) {x=3+t; y=0; label=1;};

border f(t=0, 1) {x=4; y=3*t; label=1;};

border g(t=1, 0) {x=4*t; y=3; label=1;};

border h(t=1, 0) {x=0; y=3*t; label=1;};

mesh th = buildmesh(a(10) + b(15) + c(20) + d(15) + e(10) + f(20) + g(30) + h(20));

in my geometry i want to have an arc between the borders a and e instead of b,c and,d what should i do?