 
                            ...
| Code Block | ||||
|---|---|---|---|---|
| 
 | ||||
| int id_O; //* (capital letter O) */ | 
In file bar.h:
| Code Block | ||||
|---|---|---|---|---|
| 
 | ||||
| int id_0; //* (numeric letter zero) */ | 
If a file foobar.c includes both foo.h and bar.h, then both id_0 and id_0 come in the same scope, violating this rule.
...