anchor.diff:  
       Map<String, ByteBuffer> payload = <String, ByteBuffer>{};
   
  -    PayloadPng(CanvasElement this.imageSource) {
  +    bool saveTransparency;
  +
  +    PayloadPng(CanvasElement this.imageSource, [bool this.saveTransparency = true]) {
   
       }
   
  
positive.code:             ..appendInt32(imageSource.width)
              ..appendInt32(imageSource.height)
              ..appendByte(8) // 8 bits per channel
              ..appendByte(2)//6) // truecolour with alpha
              ..appendByte(0) // compression mode 0, as per spec
              ..appendByte(0) // filter mode 0, as per spec
              ..appendByte(0) // no interlace
          ;
  
          writeDataToBlocks(builder, "IHDR", ihdr.toBuffer());
      }
  
      //################################## block writing methods
  
negative.code:         }
      }
  
      //################################## image
  
      ByteBuffer processImage() {
          
      }
  
      //################################## test stuff
  
      void testBlockWriting(ImprovedByteBuilder builder) {
  