Structure of BACE1 in complex with N-(3-((4R,5R,6S)-2-amino-6-(1,1-difluoroethyl)-5-fluoro-4-methyl-5,6-dihydro-4H-1,3-oxazin-4-yl)-4-fluorophenyl)-5-(fluoromethoxy)pyrazine-2-carboxamide