SourceDirectorySet API and General Documentation

http://www.gradle.org/docs/current/javadoc/org/gradle/api/file/SourceDirectorySet.html

What are the concrete classes that implement SourceDirectorySet Interface?

How do you instantiate a SourceDirectorySet? Where in the API documentation is this defined, if at all?

You cannot currently instantiate a source directory set, at least not using public APIs. A new model that supports this is being worked on.