public struct Board { // ... public init(width: Int, height: Int, disks: [Disk?]? = nil) { precondition(width >= 2, "`width` must be >= 2: \(width)") precondition(height >= 2, "`height` must be >= 2: \(height)") precondition(width.isMultiple(of: 2), "`width` must be an even number: \(width)") precondition(height.isMultiple(of: 2), "`height` must be an even number: \(height)")