///withoutPrompt 
function []=plot3d_tcl()
  global('a','b');
  a_new=evstr(TCL_GetVar("a"));
  b_new=evstr(TCL_GetVar("b"));
  if a<>a_new | b<>b_new then 
    a=a_new ; b = b_new ;
    t=linspace(0,2,200);
    xbasc();t=linspace(-b*%pi,b*%pi,20);plot3d(t,t,a*sin(t)'*cos(t));
  end
endfunction ///\withPrompt{}