ExternalEntityRef(p, subparser, base$, sysid$, pubid$)
subparser parameter contains a parser handle created with the same callbacks and
context as the main parser and should be used to parse the external entity.
The base$ parameter is the base to use for relative system identifiers. It is set
by parser:SetBase() and may be Nil. The sysid$ parameter is the
system identifier specified in the entity declaration and is never Nil.
The pubid$ parameter is the public ID given in the entity declaration and may be
Nil.