declare #main-output has referents-allowed
    (defaulting { string-expression (, string-expression)? })?

The declare #main-output has referents-allowed declaration is the default declaration regarding referents in an OmniMark program. When a referent has been used, unless the program was declared as referents-not-allowed, the referents will be processed.

The defaulting phrase may be used to specify default values for referents which are never defined. This is the prime use of declare #main-output has referents-allowed.