To add a new interface create a new file in this directory.
The file name should be <interface number>_<interface name>.def
The interface number should be padded to three digits to aid sorting.
New interfaces should use the next free interface code rather than 
filling in gaps. This is to avoid confusion with old removed interfaces.

In the file you should have:
 * a description block that contains a description of the interface 
   for the documentation
 * a set of message blocks that define the interfaces messages
   these are structured as 
     message { TYPE, SUBTYPE, SUBTYPE_CODE, DATA_TYPE };
 * the data types for your interface in standard C code

The best bet is to have a look at some of the other interfaces and
copy what they have done

When modifying an interface try to avoid renumbering the subtype codes.
If you remove a subtype just leave a gap, this will aid in version 
compatibility.
