operator
system-identifier of entity
system-identifier of entity retrieves the system identifier of the currently-active external entity,
if any. It can be used in an external-data-entity rule or a external-text-entity rule, or in the
dynamic scope created by the execution of these rules. So, in a function called from the body of an
external-text-entity rule, system-identifier of entity will evaluate to the system identifier of
the entity that fired the rule. If the currently-active entity is a defaulted entity,
system-identifier of entity will evaluate to the system identifier of the default entity as it was
declared in the input DTD.
In an external-data-entity rule or an external-text-entity rule, the format item %eq
is a shorthand for system-identifier of entity.
If the currently-active external entity does not have a system identifier, system-identifier of entity
uses the entity's public identifier to look up a system identifier on the current module's #library shelf,
effectively as if
#library{public-identifier of}
It is an error to invoke system-identifier of entity for an entity that
was not declared with a system identifier, or whose public identifier cannot be used in tandem with
#library to resolve to a system identifier. When in doubt, system-identifier of entity should
be preceded by the entity is system test:
do when entity is system output system-identifier of entity