func foo<T: BidirectionalCollection>(_ buffer: T) -> String
    where T.Iterator.Element == UInt8,
          T.SubSequence.Iterator.Element == T.Iterator.Element
{
    if let valueStart = buffer.index(of: 0x20 /* SP */),
       let valueEnd = buffer.reversed().index(of: 0x20 /* SP */)
    {
        return String(bytes: buffer[valueStart ... valueEnd], encoding: .utf8)!
    } else {
        return ""
    }
}