The comment in `FromStr` says that any combination of upper and lower case is considered valid, but the code only allowed a set list of variations. It also had a non exhaustive list of `CONFUSING_FORMS`.
The comment and code has been changed to only consider all upper case or all lower case denominations as valid or where BTC/btc is prefixed with a lower case c, m or u. `CONFUSING_FORMS` has been changed to contain all combinations of an upper case prefix on an otherwise valid denomination.
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