XML ExternalEntityDecl
The ExternalEntityDecl element represents a general entity where the entity definition is contained externally to the current message.
The ExternalEntityDecl is a named element and has a child of type SystemId. It can also have a child of type PublicId.
An external entity declaration of the form:
<!ENTITY bookAppendix SYSTEM "appendix.txt">has
an EntityDecl element of name bookAppendix and a child element of
type SystemId with a string value of appendix.txt.