Original text:
group's rachet tree and the credentials for the members of the group, as described in tree-hashes. <del> The "transcript_hash" field contains the list of "GroupOperation" messages that led to this state. </del>  When a new member is added to the group, an existing member of the group provides the new member with a Welcome message.  The Welcome The "tree_hash" is updated to represent the current tree and credentials <del> The "transcript_hash" is updated with the data for an MLSPlaintext message encoding a group operation in two parts: </del>  This structure incorporates everything in an MLSPlaintext up to the confirmation field in the transcript that is included in that confirmation field (via the GroupContext).  The confirmation and signature fields are then included in the transcript for the next <del> operation.  The intermediate hash enables implementations to in corporate a plaintext into the transcript without having to store the </del>  whole MLSPlaintextOpAuthData structure. When a new one-member group is created (which requires no <del> GroupOperation), the "transcript_hash" field is set to an all-zero vector of length Hash.length, where the Hash algorithm is defined by the ciphersuite. </del>  6.5.

Reviewer comments:
While trying to implement , I noticed that the confirmation and transcript hash calculations in draft-06 do not actually allow a new participant to join the transcript hash. Since the value is sent, and not the value, the new joiner is missing the auth data necessary to compute the next value. To fix this and clarify terminology, this PR makes the following changes: becomes , since this is what goes into the confirmation MAC becomes is updated to refer to the
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This looks ok. If we want, merging and messages like in the TreeKEM paper would probably avoid some pain here. Merging.

Revised text:
group's rachet tree and the credentials for the members of the group, as described in tree-hashes.  <ins> The "confirmed_transcript_hash" field contains a running hash over the handshake messages that led to this state. </ins> When a new member is added to the group, an existing member of the group provides the new member with a Welcome message.  The Welcome The "tree_hash" is updated to represent the current tree and credentials  <ins> The "confirmed_transcript_hash" is updated with the data for an MLSPlaintext message encoding a group operation in two parts: </ins> This structure incorporates everything in an MLSPlaintext up to the confirmation field in the transcript that is included in that confirmation field (via the GroupContext).  The confirmation and signature fields are then included in the transcript for the next  <ins> operation.  The interim transcript hash is passed to new members in the WelcomeInfo struct, and enables existing members to incorporate a handshake message into the transcript without having to store the </ins> whole MLSPlaintextOpAuthData structure. When a new one-member group is created (which requires no  <ins> GroupOperation), the "interim_transcript_hash" field is set to the zero-length octet string. </ins> 6.5.