anchor.diff:  The `svg` element is the root element of the document.
   It's defined only once and can't be nested, unlike the SVG spec.
   
  +Always present.
  +
   Children: `defs`, `g`, `path`, `text` and `image`.
   
   Attributes: `width`, `height`, `viewBox` and `preserveAspectRatio`.
   
   === defs
  +
  +Always present.
  +
  +All `defs` children have an `id` attribute. It always set and newer empty.
   
   Children: `linearGradient`, `radialGradient`, `clipPath`, `mask`, `pattern`,
   `marker` and `filter`.
  
positive.code: 
  === image
  
  An image can have base64 encoded data or a path to an image.
  
  Attributes: `id`, `x`, `y`, `width`, `height`, `preserveAspectRatio`, `xlink:href`,
  `visibility` and `transform`.
  
negative.code: [[transform_attr]]
  
  * `transform`
  ** `transform` is always a `matrix`. Never `translate`, `scale`, etc.
  ** Can be skipped.
  * `viewBox`
  ** Can be skipped, because doesn't have a default value.
  * `preserveAspectRatio`
  ** Will be set only when `viewBox` is set.
  