enveloppe(d):={
local zM,a,b,c,a1,b1,c1,dpd1;
a:=derive(d,y);
b:=derive(d,x);
c:=subst(subst(d,x=0),y=0);
a1:=derive(a,t);b1:=derive(b,t);c1:=derive(c,t);
dpd1:=a*b1-b*a1;
if (dpd1!=0) {
  zM:=(i*(-c*b1+b*c1)+(c*a1-a*c1))/dpd1;
  return(plotparam(zM,t));
}
else return("Droites paralleles");
};