finish rename

Author: vladak

modules/opaque-structures.md

@@ -7,35 +7,35 @@ actually contains.  The reason to do that is not to allow a programmer to depend
 anytime.
 
 ```C
-/* Forward declaration in foo.h */
-struct foo;
+/* Forward declaration in opaque.h */
+struct opaque;
 
 void
-do_stuff(struct foo *f);
+do_stuff(struct opaque *f);
 ```
 
 The file implementing `do_stuff()` **will cast the opaque structure object to
 an internal structure type**.
 
 ```C
-/* in foo_impl.h not provided to the consumers */
-struct foo_impl {
+/* in opaque_impl.h not provided to the consumers */
+struct opaque_impl {
     int x;
     int y;
 };
 
 /* library implementation code */
 void
-do_stuff(struct foo *f)
+do_stuff(struct opaque *f)
 {
-	struct foo_impl *fi = (struct foo_impl *)f;
+	struct opaque_impl *fi = (struct opaque_impl *)f;
 
 	fi->x = ...
 	fi->y = ...
 }
 ```
 
-Then any `.c` file that includes `foo.h` can work with the structure
+Then any `.c` file that includes `opaque.h` can work with the structure
 (by passing it to `do_stuff()`) however cannot access its members directly.
 The structure is usable only via a pointer. Thus, the library has to provide
 a function to allocate a structure and return the pointer to it
@@ -45,20 +45,20 @@ inside the structure.
 This is handy for libraries so they are free to change the layout of structures
 without breaking the consumers.
 
-The consumer of the library then `#include`s only `foo.h` but it does not have
-access to `foo_impl.h`.
+The consumer of the library then `#include`s only `opaque.h` but it does not have
+access to `opaque_impl.h`.
 
 ```C
 #include <stdio.h>
 
-#include "foo.h"
+#include "opaque.h"
 
 int
 main(void)
 {
-	struct foo *h;
+	struct opaque *h;
 
-	h = getFoo();
+	h = getOpaque();
 	doStuff(h);
 }
 ```