anchor.diff:  which--not too surprisingly--has greetings as its main purpose. 
   
   [source,c]
  -.foo.h
  +.`foo.h`
   ----
   #ifndef FOO_H
   #define FOO_H
  
positive.code: ----
  
  [source,c]
  .foo.c
  ----
  
  #include "foo.h"
  
negative.code: and test it using a quick C executable:
  
  [source,c]
  .`foo_test.c`
  ----
  #include "foo.h"
  
  