Although reviewed portions of Hales's proof seemed correct, its sprawling computer-code calculations made it difficult to determine whether the complete proof established the claimed theorem.

“Every part of the proof that reviewers looked at seemed correct, but the proof as a whole was such a sprawling edifice of calculations, buried deep in computer code, and it was hard to determine how they all hung together and whether, in the end, they yielded the proof Hales claimed they did.”