   /// Given an ill-formed sequence, find the length of its maximal subpart.
   @inline(never)
   @warn_unused_result
-  static func _findMaximalSubpartOfIllFormedUTF8Sequence(
+  internal static func _findMaximalSubpartOfIllFormedUTF8Sequence(
       buffer: UInt32, validBytes: UInt8) -> UInt8 {
     var buffer = buffer
     var validBytes = validBytes


  /// Lookahead buffer used for UTF-8 decoding.  New bytes are inserted at LSB,
  /// and bytes are read at MSB.
  var _decodeLookahead: UInt32 = 0

  /// Flags with layout: `0bxxxx_yyyy`.
  ///

  /// Returns `true` if the LSB bytes in `buffer` are well-formed UTF-8 code
  /// unit sequence.
  @warn_unused_result
  internal static func _isValidUTF8(buffer: UInt32, validBytes: UInt8) -> Bool {
    _sanityCheck(validBytes & 0b0000_1111 != 0,
        "input buffer should not be empty")

