Browse[2]> unnest(list(a = 1:3, b = numeric()), s(stack = "ix", as = "point", s(stack = "id")))
ix point
1 a 1
2 a 2
3 a 3
4 b NA
Browse[2]> unnest(list(a = as.list(1:3), b = list()), s(stack = "ix", as = "point", s(stack = "id")))
ix point point.id
1 a 1 1
2 a 2 2
3 a 3 3
4 b NA NA
Browse[2]>