anchor.diff:     _dependency1_ _dependency2_+.  (See <<ref_dependencies,the reference on
      dependency types>>.)
   
  -
   3. Variable declarations, which look like +_variable_ = _value_+.
   
   4. References to more files, which look like +subninja _path_+ or
  
positive.code: 
  4. Variables from the file that included that file using the
     `subninja` keyword.
  
  Future work
  -----------
  
negative.code: delete a depfile-discovered header file and rebuild, without the build
  aborting due to a missing input.
  
  `description`:: a short description of the command, used to pretty-print
    the command as it's running.  The `-v` flag controls whether to print
    the full command or its description; if a command fails, the full command
  