purge(t);
  purge(tt);
  purge(x);
  purge(y);
  xyztrange(-6.0,6.0,-7.0,4.0,-10.0,10.0,-3.0,3.0,-6.0,6.0,-2.0,4.0,1);
  a:=cos(t)*(1-cos(2*t));
  b:=sin(t)*cos(2*t);
  c:=-sin(t)*cos(t);
  enveloppe3(a,b,c);
  tt:=element(-3..4);
  aa:=subst(a,t,tt);
  bb:=subst(b,t,tt);
  cc:=subst(c,t,tt);
  plotfunc((-bb*x-cc)/aa,x);