anchor.diff:  where `target` is a known output described by `build.ninja` in the
   current directory.
   
  +
   Creating .ninja files
   ~~~~~~~~~~~~~~~~~~~~~
  -A build file (default name: `build.ninja`) provides a list of _rules_
  -along with a list of _build_ statements saying how to build files
  -using the rules.  The only other syntax is a way of declaring
  -(immutable) variables.
  -
  -Conceptually, `build` statements describe the dependency graph of your
  -project, while `rule` statements describe how to generate the files
  -along a given edge of the graph.
  -
   Here's a basic `.ninja` file that demonstrates most of the syntax.
   
   ---------------------------------
  
positive.code: The basic example above declares a new rule named `cc`, along with the
  command to run.  (In the context of a rule, the `command` key is
  special and defines the command to run.  A full list of special keys
  is provided in the reference.)
  
  Variables begin with a `$` and are described more fully later, but
  here `$in` expands to the list of input files (`foo.c`) and `$out` to
  
negative.code: 
  Rule variables
  ~~~~~~~~~~~~~~
  [[ref_rule]]
  
  A `rule` block contains a list of `key = value` declarations that
  affect the processing of the rule.  Here is a full list of special
  