In SGML, attributes may be of type entity or entities. Retrieving the data associated with an entity that is the value of an attribute is similar to retrieving the value of a regular entity, except that you must do so in an element rule instead of an external text or data entity rule.

This has the following consequences:

• You need to determine whether the attribute is an entity (or entities) attribute using the is entity (or is entities) test.
• You need to determine whether the entity is external or internal using the is external (or is internal) test, and act accordingly.
• You use the format items "%pv" and "%ev" instead of "%pq" and "%eq" to access the PUBLIC and SYSTEM identifiers. For an internal entity, you can use the format item "%ev" to access the replacement value of the attribute.
• The entity is in-library test is not available, so you must use the test #library has key "%pv(attribute-name)" instead.

  declare catch entity-replacement-found
  element "foo"
     repeat over attributes as this-attribute
        do when attribute this-attribute is entity
           do when attribute this-attribute is external
              do when #library has key "%pv(baz)"
                 do when file #library{"%pv(baz)"} exists
                    output file #library{"%pv(baz)"}
                 else
                    repeat over #libpath
                       do when file (#libpath || #library{"%pv(baz)"}) exists
                          output file (#libpath || #library{"%pv(baz)"})
                          throw entity-replacement-found
                       done
                    again
                 done
              else
                 do when file "%ev(baz)" exists
                    output file "%ev(baz)"
                    throw entity-replacement-found
                 done
              done
              output ""
              catch entity-replacement-found
           else ;it is an internal entity
              output "%ev(this-attribute)" ; output its replacement text
           done
        done
     again
     output "%c"

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