Other `Recursive`

If I understand right, `r(x) = modify(f, x, Recursive(p, o))` means "for each element `e` of `o(x)`, if `p(e)` then `r(e)` else `f(e)`."

But I think that means I can't say "recurse on `x.children` if `hasproperty(x, :children)` and call `f(x)`".

Anybody run into this kind of problem or have thoughts about it?