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");
  };