This is ugly API for now, but it's a simple and relatively safe
solution. It should be cleaned up later.
Data from the Config object can only be accessed by opening the "root"
for reading (OpenRead) or writing (OpenWrite). Multiple readers may be
open simultaneously, but only one writer, which guarantees atomic
behavior. There are ugly edge-cases for save errors and pointer-style
objects in the config tree, so use good behavior.
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