well it would be a lot nicer if
public static
let IHDR:Tag = .init(73, 72, 68, 82)
was
public static
let IHDR:Tag = .init('I', 'H', 'D', 'R')
or just
public static
let IHDR:Vector4<UInt8> = ('I', 'H', 'D', 'R')
(coming soon to a swift evolution near you!)