Set the system identifier for this Source. If an input source has already been set, it will set the system ID or that input source, otherwise it will create a new input source.
The system identifier is optional if there is a byte stream or a character stream, but it is still useful to provide one, since the application can use it to resolve relative URIs and can include it in error messages and warnings (the parser will attempt to open a connection to the URI only if no byte stream or character stream is specified).