anchor.diff:    the full command or its description; if a command fails, the full command
     line will always be printed before the command's output.
   
  +`in`, `out`:: the space-separated list of inputs and outputs to the `build`
  +  line referencing this `rule`.
  +
   Evaluation and scoping
   ~~~~~~~~~~~~~~~~~~~~~~
   XXX talk about where variables live, nested scopes etc
  
positive.code: 
  Rule variables
  ~~~~~~~~~~~~~~
  
  A `rule` block contains a list of `key = value` declarations that
  affect the processing of the rule.  Here is a full list of special
  
negative.code: where `target` is a known output described by `build.ninja` in the
  current directory.
  
  
  Creating .ninja files
  ~~~~~~~~~~~~~~~~~~~~~
  Here's a basic `.ninja` file that demonstrates most of the syntax.
  
  ---------------------------------
  