* flag, so that it can be interleaved with the other walker.
* If a none block is passed, starts at the block this node belongs to.
* If end is passed also visits kept alive blocks. Does not use the link field.
* flag, so that it can be interleaved with the other walker.
* If a none block is passed, starts at the block this node belongs to.
* If end is passed also visits kept alive blocks. Does not use the link field.