Bitmask for extracting a valid 31-bit BIP32 child index. BIP32 child indices
are 32-bit unsigned integers where 0 to 2^31-1 is for normal derivation and
2^31 to 2^32-1 is for hardened derivation. This mask ensures derived indices
stay in the valid range (0 to 2,147,483,647).
Bitmask for extracting a valid 31-bit BIP32 child index. BIP32 child indices are 32-bit unsigned integers where 0 to 2^31-1 is for normal derivation and 2^31 to 2^32-1 is for hardened derivation. This mask ensures derived indices stay in the valid range (0 to 2,147,483,647).
https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki