select, reject

[frankshearar]

This requires #14, because otherwise we will move to an unpredictable point in the traversal. Otherwise, the idea's simple:

def select(&predicate)
  map { |node|
    @zipper.remove unless predicate.call(node)
  }
end

def reject(&predicate)
  map { |node|
    @zipper.remove if predicate.call(node)
  }
end