anchor.diff:  Now, let's say you want to compile your basic C hello world using
   P-Redo. Let's say you have an +hello.c+ like this:
   
  -[source,c]
   ----
   #include <stdio.h>
   
  
positive.code: To compile it to an object file you should create a +default.c.o+ like
  this:
  
  [source,python,numbered]
  ----
  redo.if_changed(basename) <1>
  redo.utils.cmd(["gcc", "-c", "-o", basename + ".o", basename]) <2>
  
negative.code: build-script I will use a specific one and create this +hello.do+
  script file:
  
  ----
  redo.if_changed("hello.c.o")
  redo.utils.cmd("gcc -o hello hello.c.o")
  