///withoutPrompt 
function draw_chain_from_angles(a,r,job,rt)
// a the angles , a(i,j) is the angle of node i at time t(j)
// r the segments half length
  if argn(2)<3 then job=0,end
  if argn(2)<4 then rt = 0.01;end 
  n2=size(a,2);
  x=[0*ones(1,n2);cumsum(2*diag(r)*cos(a),1)];
  y=[0*ones(1,n2);cumsum(2*diag(r)*sin(a),1)];
  draw_chain_from_coordinates(x,y,job,rt)
endfunction

function draw_chain_from_coordinates(x,y,job,rt)
// x(i,j), y(i,j) is the coordinate of node i at time t(j)
// r the segments half length
  if argn(2)<3 then job=0,end
  if argn(2)<4 then rt = 0.01;end 
  [n1,n2]=size(x);
  b = maxi(maxi(abs(x)),maxi(abs(y)));
  xbasc();set figure_style new;  /// \sleftarrow{\normalfont new graphics mode }
  a=gca()  /// \sleftarrow{\normalfont we want to set }
  a.data_bounds=[-1,-1;1,0]*b;/// \sleftarrow{\normalfont the graphics boundaries }
  a.isoview="on";/// \sleftarrow{\normalfont and use an isoview mode }
  xset('pixmap',1); /// \sleftarrow{\normalfont double buffer mode }
  drawlater() /// \sleftarrow{\normalfont wait {for} graphics display }
  colors= 1:n1-1; colors(8)=n1;
  xsegs([x(1:$-1,1)';x(2:$,1)'],[y(1:$-1,1)';y(2:$,1)'],colors) /// \sleftarrow{\normalfont draw}
  p=gce();p.thickness=4; /// \sleftarrow{\normalfont \verb+p+ is used to keep track of \verb+xsegs+ data }
  if job==1 then
     xpoly(x($,1)*ones(2,1),y($,1)*ones(2,1),'lines'); /// \sleftarrow{\normalfont  {end}point trajectory}
     t=gce();t.line_style=2;
  end
  drawnow()  /// \sleftarrow{\normalfont perform drawing }
  xset('wshow') /// \sleftarrow{\normalfont update display with double buffer }
  ind=[1;(2:n1-1)'.*.ones(2,1);n1]
  realtimeinit(rt); /// \sleftarrow{\normalfont set time unit: smaller values will accelerate the animation  }
  for j=1:n2,
    realtime(j) /// \sleftarrow{\normalfont wait to time \verb+j+ before continuing  }
    drawlater() 
    p.data = [x(ind,j),y(ind,j)]; /// \sleftarrow{\normalfont update \verb+xsegs+ data }
    if job==1 then t.data=[t.data;[x($,j),y($,j)]],end/// \sleftarrow{\normalfont update \verb+xpoly+ data }
    drawnow() /// \sleftarrow{\normalfont since objects are updated,  graphics is cleared and redrawn }
    xset('wshow') /// \sleftarrow{\normalfont update display with double buffer }
  end
endfunction ///\withPrompt{}