XML SystemId
SystemId is a value element that represents a system identifier in an XML message.
SystemId can be part of a DocTypeDecl, NotationDecl, or
UnparsedEntityDecl element. The value of the SystemId is a URI, and
is typically a URL or the name of a file on the current system. A
system identifier of the form SYSTEM "Note.dtd"
has
a string value of Note.dtd
.