Even though SIDL is the primary input format for Babel, it is not the only format Babel understands. For type repositories (similar in function to include directories for C/C++ headers) the preferred language to articulate types is XML.
Babel has the capabilities to convert SIDL files into XML files adhering to the SIDL.DTD. The XML files in these repositories can be included in subsequent runs quickly since all the external references were resolved by Babel during their creation. A SIDL file may refer to unresolved types.
Similarly, Babel can convert XML files that adhere to the SIDL.DTD to SIDL files.