docs: public final void putKey(DeterministicKey key) {
          ImmutableList<ChildNumber> path = key.getPath();
          // Update our tracking of what the next child in each branch of the tree should be. Just assume that keys are
          // inserted in order here.
          final DeterministicKey parent = key.getParent();
          if (parent != null)
              lastChildNumbers.put(parent.getPath(), key.getChildNumber());
          keys.put(path, key);
      }