     let bit0 = (lut0 >> index) & 1
     let bit1 = (lut1 >> index) & 1
 
-    // Note that all past-EOF bytes are 0 and thus never a valid continuation.
     switch (bit1, bit0) {
     case (0, 0): // 2-byte sequence.
       // Require 10xx xxxx  110x xxxx.

    // The first byte to read is located at MSB of `_decodeLookahead`.  Get a
    // representation of the buffer where we can read bytes starting from LSB.
    var buffer = _decodeLookahead.byteSwapped
    if _slowPath(!UTF8._isValidUTF8(buffer, validBytes: _lookaheadFlags)) {
      // The code unit sequence is ill-formed.  According to Unicode
      // recommendation, replace the maximal subpart of ill-formed sequence
      // with one replacement character.


    _sanityCheck(validBytes != 0,
        "input buffer should not be empty")
    _sanityCheck(!UTF8._isValidUTF8(buffer),
        "input sequence should be ill-formed UTF-8")

    // Unicode 6.3.0, D93b:
