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);