static L<DoublePt> toDoublePts(L<Pt> l) {
  ret map toDoublePt(l);
}

static L<DoublePt> toDoublePts(double... coords) {
  int n = l(coords)/2;
  L<DoublePt> l = emptyList(n);
  for i to n:
    l.add(doublePt(coords[i*2], coords[i*2+1]));
  ret l;
}