Change:
This clears up the wording around how to prune the ratchet tree on a Remove operation. Previously, it could be interpreted as "remove nodes from the tree until the rightmost node is not null", but this is not what is intended by the authors, and it also has the ability to produce invalid trees (i.e., trees with an even number of elements). The second change is to do the truncation after doing the Blank propogation up the direct path of the removed leaf. Note that the order of these steps doesn't matter in that they produce the same resulting tree. However, if we blank after pruning, an implementor would have to take care to not to try to blank out previously-removed nodes. Prune-then-blank is a little easier to reason about. The third change is a semantic fix. It is clear from the last paragraph in this section that the root node should be blanked, but the text only said to blank the direct path of the leaf, which does not include the root node. I just added that part in.
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Discussion:
8.4. A Remove message is sent by a group member to remove one or more <del> members from the group. </del> <ins> other members from the group.  A member MUST NOT use a Remove message to remove themselves from the group.  If a member of a group receives a Remove message where the removed index is equal to the signer index, the recipient MUST reject the message as malformed. </ins> The sender of a Remove message generates it as as follows:

Update the ratchet tree by replacing nodes in the direct path from the removed leaf using the information in the Remove message <del> Reduce the size of the roster and the tree until the rightmost element roster element and leaf node are non-null </del> Update the ratchet tree by setting to blank all nodes in the <del> direct path of the removed leaf </del> <ins> direct path of the removed leaf, and also setting the root node to blank Truncate the roster such that the last roster element is non-null Truncate the tree such that the rightmost non-blank leaf is the last node of the tree Note that, in step 4, there must be at least one non-null element in the roster, since any valid GroupState must have the current member in the roster and self-removal is prohibited.  The same reasoning justifies the existence of a non-blank leaf in the ratchet tree in step 5. </ins> The update secret resulting from this change is the path secret computed for the root node of the ratchet tree in the first step.