Bitcoin mining uses SHA-256d: SHA-256(SHA-256(data)).I recently discovered experimentally (IACR ePrint 2026/109079) thatthe second SHA-256 application has a structurally constrained messageschedule:The second hash always receives exactly 32 bytes (the first hashoutput) + fixed Merkle-Damgård paddingThis makes W[8-15] in the second hash always constant(0x80000000... + length encoding)Only 30 unique carry count values exist in the second hash'sW-schedule (constrained Binomial distribution due to constant input)Measurable cross-hash carry anti-correlation: r=−0.029, 6.5σ,N=50K (internal state — first hash carry count negatively predictssecond hash carry count)My questions:Was this structural property of SHA-256d considered when Bitcoinadopted double-SHA-256? Or was it chosen purely for length-extensionattack resistance?Is there any documentation of this constrained W-schedule effect inBitcoin's design rationale?Does this property have any known implications for Bitcoin's securitymodel beyond length-extension resistance?The carry correlation is not exploitable (r=−0.029,