anchor.diff:    /// Returns a [Uint8List] of the ASCII codes of the encoded data.
     ///
     /// If the input, including left over [_state] from earlier encodings,
  -  /// are not a multiple of three bytes, then the partial state is stored
  +  /// is not a multiple of three bytes, then the partial state is stored
     /// back into [_state].
     /// If [isLast] is true, partial state is encoded in the output instead,
     /// with the necessary padding.
  
positive.code:   /// only call this function after having seen at least one `=` or `%`
    /// character.
    /// If the number of missing characters is not 3 or 0, we have seen (at least)
    /// a `%` character and expects the rest of the `%3D` sequence, and a `=` is
    /// not allowed. When missing 3 characters, either `=` or `%` is allowed.
    ///
    /// When the value is 0, no more padding (or any other character) is allowed.
  
negative.code:   /// it needs when input is valid, and at least enough bytes to reach the error
    /// when input is invalid.
    ///
    /// Never count more than two padding sequences as any more than that
    /// will raise an error anyway, and we only care about being precise for
    /// successful conversions.
    static int _trimPaddingChars(String input, int start, int end) {
  