def encode(in_bytes):
    """Encode a string using Consistent Overhead Byte Stuffing (COBS).
    
    Input is any byte string. Output is also a byte string.
    
    Encoding guarantees no zero bytes in the output. The output
    string will be expanded slightly, by a predictable amount.
    
    An empty string is encoded to '\\x01'"""
    final_zero = True
    out_bytes = []
    idx = 0
    search_start_idx = 0
    for in_char in in_bytes:
        if in_char == '\x00':
            final_zero = True
            out_bytes.append(chr(idx - search_start_idx + 1))
            out_bytes.append(in_bytes[search_start_idx:idx])
            search_start_idx = idx + 1
        elif idx - search_start_idx == 253:
            final_zero = False
            out_bytes.append('ÿ')
            out_bytes.append(in_bytes[search_start_idx:idx + 1])
            search_start_idx = idx + 1
        idx += 1
    if idx != search_start_idx or final_zero:
        out_bytes.append(chr(idx - search_start_idx + 1))
        out_bytes.append(in_bytes[search_start_idx:idx])
    return ''.join(out_bytes)
