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BIP 390: Add MuSig2 descriptor BIP
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‎README.mediawiki

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| Informational
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| Draft
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|-
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| [[bip-0390.mediawiki|390]]
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| Applications
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| musig() Descriptor Key Expression
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| Ava Chow
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| Informational
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| Draft
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|-
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| [[bip-0431.mediawiki|431]]
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| Applications
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| Topology Restrictions for Pinning

‎bip-0380.mediawiki

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|-
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| <tt>tr(KEY)</tt>, <tt>tr(KEY, TREE)</tt>
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| [[bip-0386.mediawiki|386]]
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|-
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| <tt>musig(KEY, KEY, ..., KEY)</tt>
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| [[bip-0390.mediawiki|390]]
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|}

‎bip-0390.mediawiki

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<pre>
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BIP: 390
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Layer: Applications
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Title: musig() Descriptor Key Expression
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Author: Ava Chow <[email protected]>
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Comments-Summary: No comments yet.
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Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-0390
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Status: Draft
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Type: Informational
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Created: 2024-01-15
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License: CC0-1.0
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</pre>
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==Abstract==
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This document specifies a <tt>musig()</tt> key expression for output script descriptors.
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<tt>musig()</tt> expressions take multiple keys and produce an aggregate public key using BIP 327.
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==Copyright==
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This BIP is licensed under the Creative Commons CC0 1.0 Universal license.
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==Motivation==
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BIP 327 introduces the MuSig2 Multi-Signature scheme. It is useful to have a way for keys to be used
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in a MuSig2 aggregate key to be expressed in descriptors so that wallets can more easily use MuSig2.
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==Specification==
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A new key expression is defined: <tt>musig()</tt>.
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===<tt>musig(KEY, KEY, ..., KEY)</tt>===
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The <tt>musig(KEY, KEY, ..., KEY)</tt> expression can only be used inside of a <tt>tr()</tt>
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expression as a key expression. It additionally cannot be nested within another <tt>musig()</tt>
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expression. Repeated participant public keys are not allowed. The aggregate public key is produced
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by using the <tt>KeyAgg</tt> algorithm on all KEYs specified in the expression after performing all
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specified derivation. As with script expressions, KEY can contain child derivation specified by
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<tt>/*</tt>. A new aggregate public key will be computed for each child index. Keys must be sorted
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with the <tt>KeySort</tt> algorithm after all derivation and prior to aggregation<ref>'''Why must
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the keys be sorted prior to aggregation?''' Although the descriptor's written form sets an order
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for the keys that could be used for aggregation, the order should not matter as MuSig2 philosophically
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operates over a set of keys, with the order merely being an implementation detail in aggregation
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itself. Requiring sorting of keys prior to aggregation enforces this philosophy as keys can be
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written in the descriptor in any order with the end result still being the same. Furthermore, this
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aids with recovery where the descriptor was not backed up as users will not need to also have
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backed up, or guess, the correct order of keys.</ref>.
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===<tt>musig(KEY, KEY, ..., KEY)/NUM/.../*</tt>===
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<tt>musig(KEY, KEY, ..., KEY)/NUM/.../*</tt> expressions are also allowed, with the same usage
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restrictions as in the previous section. The aggregate public key
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is first computed as described above, with the keys also being sorted after all derivation and prior
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to aggreation. Then further BIP 32 derivation will be performed on the aggregate public key as described in
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[[bip-0328.mediawiki|BIP 328]]. As there is no aggregate private key,
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only unhardened derivation from the aggregate public key is allowed, and thus the derivation steps
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following the <tt>musig()</tt> expression cannot contain
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<tt>/NUMh</tt> or <tt>/NUM'</tt> derivation steps nor <tt>/*h</tt>, or <tt>/*'</tt> child derivation.
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For these <tt>musig()</tt> expressions, the KEY expressions contained within must be xpubs or derived from
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xpubs, and cannot contain child derivation as specified by a <tt>/*</tt>, <tt>/*'</tt>, or <tt>/*h</tt>.
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==Test Vectors==
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Valid descriptors containing followed by the scripts they produce. Descriptors involving derived child keys
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will have the 0th, 1st, and 2nd scripts listed.
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* <tt>rawtr(musig(KwDiBf89QgGbjEhKnhXJuH7LrciVrZi3qYjgd9M7rFU74sHUHy8S,03dff1d77f2a671c5f36183726db2341be58feae1da2deced843240f7b502ba659,023590a94e768f8e1815c2f24b4d80a8e3149316c3518ce7b7ad338368d038ca66))</tt>
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** <tt>5120789d937bade6673538f3e28d8368dda4d0512f94da44cf477a505716d26a1575</tt>
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* <tt>tr(musig(02f9308a019258c31049344f85f89d5229b531c845836f99b08601f113bce036f9,03dff1d77f2a671c5f36183726db2341be58feae1da2deced843240f7b502ba659,023590a94e768f8e1815c2f24b4d80a8e3149316c3518ce7b7ad338368d038ca66))</tt>
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** <tt>512079e6c3e628c9bfbce91de6b7fb28e2aec7713d377cf260ab599dcbc40e542312</tt>
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* <tt>rawtr(musig(xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y)/0/*)</tt>
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** <tt>51209508c08832f3bb9d5e8baf8cb5cfa3669902e2f2da19acea63ff47b93faa9bfc</tt>
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** <tt>51205ca1102663025a83dd9b5dbc214762c5a6309af00d48167d2d6483808525a298</tt>
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** <tt>51207dbed1b89c338df6a1ae137f133a19cae6e03d481196ee6f1a5c7d1aeb56b166</tt>
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* <tt>tr(musig(xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y)/0/*,pk(f9308a019258c31049344f85f89d5229b531c845836f99b08601f113bce036f9))</tt>
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** <tt>51201d377b637b5c73f670f5c8a96a2c0bb0d1a682a1fca6aba91fe673501a189782</tt>
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** <tt>51208950c83b117a6c208d5205ffefcf75b187b32512eb7f0d8577db8d9102833036</tt>
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** <tt>5120a49a477c61df73691b77fcd563a80a15ea67bb9c75470310ce5c0f25918db60d</tt>
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* <tt>tr(f9308a019258c31049344f85f89d5229b531c845836f99b08601f113bce036f9,pk(musig(xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y)/0/*))</tt>
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** <tt>512068983d461174afc90c26f3b2821d8a9ced9534586a756763b68371a404635cc8</tt>
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** <tt>5120368e2d864115181bdc8bb5dc8684be8d0760d5c33315570d71a21afce4afd43e</tt>
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** <tt>512097a1e6270b33ad85744677418bae5f59ea9136027223bc6e282c47c167b471d5</tt>
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Invalid descriptors
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* <tt>musig()</tt> is not allowed in <tt>pk()</tt>: <tt>pk(musig(02f9308a019258c31049344f85f89d5229b531c845836f99b08601f113bce036f9,03dff1d77f2a671c5f36183726db2341be58feae1da2deced843240f7b502ba659,023590a94e768f8e1815c2f24b4d80a8e3149316c3518ce7b7ad338368d038ca66))</tt>
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* <tt>musig()</tt> is not allowed in <tt>pkh()</tt>: <tt>pkh(musig(02f9308a019258c31049344f85f89d5229b531c845836f99b08601f113bce036f9,03dff1d77f2a671c5f36183726db2341be58feae1da2deced843240f7b502ba659,023590a94e768f8e1815c2f24b4d80a8e3149316c3518ce7b7ad338368d038ca66))</tt>
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* <tt>musig()</tt> is not allowed in <tt>wpkh()</tt>: <tt>wpkh(musig(02f9308a019258c31049344f85f89d5229b531c845836f99b08601f113bce036f9,03dff1d77f2a671c5f36183726db2341be58feae1da2deced843240f7b502ba659,023590a94e768f8e1815c2f24b4d80a8e3149316c3518ce7b7ad338368d038ca66))</tt>
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* <tt>musig()</tt> is not allowed in <tt>combo()</tt>: <tt>combo(musig(02f9308a019258c31049344f85f89d5229b531c845836f99b08601f113bce036f9,03dff1d77f2a671c5f36183726db2341be58feae1da2deced843240f7b502ba659,023590a94e768f8e1815c2f24b4d80a8e3149316c3518ce7b7ad338368d038ca66))</tt>
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* <tt>musig()</tt> is not allowed in <tt>sh(wpkh())</tt>: <tt>sh(wpkh(musig(02f9308a019258c31049344f85f89d5229b531c845836f99b08601f113bce036f9,03dff1d77f2a671c5f36183726db2341be58feae1da2deced843240f7b502ba659,023590a94e768f8e1815c2f24b4d80a8e3149316c3518ce7b7ad338368d038ca66)))</tt>
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* <tt>musig()</tt> is not allowed in <tt>sh(wsh())</tt>: <tt>sh(wsh(pk(musig(02f9308a019258c31049344f85f89d5229b531c845836f99b08601f113bce036f9,03dff1d77f2a671c5f36183726db2341be58feae1da2deced843240f7b502ba659,023590a94e768f8e1815c2f24b4d80a8e3149316c3518ce7b7ad338368d038ca66))))</tt>
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* <tt>musig()</tt> is not allowed in <tt>wsh()</tt>: <tt>wsh(musig(02f9308a019258c31049344f85f89d5229b531c845836f99b08601f113bce036f9,03dff1d77f2a671c5f36183726db2341be58feae1da2deced843240f7b502ba659,023590a94e768f8e1815c2f24b4d80a8e3149316c3518ce7b7ad338368d038ca66))</tt>
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* <tt>musig()</tt> is not allowed in <tt>sh()</tt>: <tt>sh(musig(02f9308a019258c31049344f85f89d5229b531c845836f99b08601f113bce036f9,03dff1d77f2a671c5f36183726db2341be58feae1da2deced843240f7b502ba659,023590a94e768f8e1815c2f24b4d80a8e3149316c3518ce7b7ad338368d038ca66))</tt>
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* Ranged <tt>musig()</tt> requires all participants to be xpubs: <tt>tr(musig(02f9308a019258c31049344f85f89d5229b531c845836f99b08601f113bce036f9,03dff1d77f2a671c5f36183726db2341be58feae1da2deced843240f7b502ba659,023590a94e768f8e1815c2f24b4d80a8e3149316c3518ce7b7ad338368d038ca66)/0/0)</tt>
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* Cannot have ranged participants if <tt>musig()</tt> is also ranged: <tt>tr(musig(xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL/*,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y)/0/*)</tt>
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* <tt>musig()</tt> cannot have hardened derivation steps: <tt>tr(musig(xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y)/0h/*)</tt>
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* <tt>musig()</tt> cannot have hardened child derivation: <tt>tr(musig(xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y)/0/*h)</tt>
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==Backwards Compatibility==
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<tt>musig()</tt> expressions use the format and general operation specified in
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[[bip-0380.mediawiki|BIP 380]]. As these are a set of wholly new expressions, they are not compatible
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with any implementation. However the keys are produced using a standard process so existing software
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are likely to be familiar with them.
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==Rationale==
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<references/>
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==Reference Implementation==
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TBD
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==Acknowledgements==
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Thanks to Pieter Wuille, Andrew Poelstra, Sanket Kanjalkar, Salvatore Ingala, and all others who
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participated in discussions on this topic.

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