% given a triangle (a,b,c) and a radius r, plot a circular arc
% of radius r centered at a that connected edges ab and ac
function plot_prop_arc(a,b,c,r)
       angle0=imag(log(c-a));
       angle1=imag(log((b-a)/(c-a) ));
       angles=angle0+[0:.025:1]*angle1;
       z=a+r*exp(i*angles);
       plot(real(z),imag(z),'k')
return 