Every way a mnemonic could be invalid was reported as a checksum
failure, including an empty one. It is now refused as empty, as having
a word count other than 12, 15, 18, 21 or 24, as having a word that is
not on the BIP-39 word list, or as failing the checksum. A word not on
the list is named by its position, never by the word itself.
Model: opus-5-5
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