function drawXu(X,u,L,is)
  clf; view(-30,45); [NX,nd]=size(X);
  [nL,nseg]=size(L); x=[]; y=[]; z=[];
  U=reshape(u,3,NX)';
  for k=1:is:nL
    x=[x X(L(k,1),1)+U(L(k,1),1) X(L(k,2),1)+U(L(k,2),1)];
    y=[y X(L(k,1),2)+U(L(k,1),2) X(L(k,2),2)+U(L(k,2),2)];
    z=[z X(L(k,1),3)+U(L(k,1),3) X(L(k,2),3)+U(L(k,2),3)];
  end;
  plot3(x,y,z);
end
