anchor.diff:  * <<constant-keyword,`constant_keyword`>>
   * <<text,`text`>> field with a <<keyword,`keyword`>> or
     <<constant-keyword,`constant_keyword`>> sub-field
  -
  -Fields containing <<array,array values>> use the first array item only.
   --
   
   `<substring>`::
  
positive.code: * <<constant-keyword,`constant_keyword`>>
  * <<text,`text`>> field with a <<keyword,`keyword`>> or
    <<constant-keyword,`constant_keyword`>> sub-field
  
  Fields containing <<array,array values>> use the first array item only.
  --
  
  `<left>`::
  
negative.code: * <<constant-keyword,`constant_keyword`>>
  * <<text,`text`>> field with a <<keyword,`keyword`>> or
    <<constant-keyword,`constant_keyword`>> sub-field
  --
  
  `<substring>`::
  