function demo_pendulum()
  npend_build_and_load() ;
  n=np(); r=ones(1,n); m=ones(1,n); j=ones(1,n); g=9.81;
  y0=0*ones(2*n,1);tt=0:0.05:10;
  yt=ode(y0,0,tt,'npend');
  draw_chain_from_angles(yt(1:$/2,:),r,1);
endfunction