define external output function function-name argument-list
    as string-expression
     (in function-library string-expression)?

Defines an external output stream.

Streams attached to external output functions support all of the usual stream operations: open, reopen, put, close, has name, and name of. Since the external output function is not required to provide a name, the has name test can be used to ensure that the name is present before trying to retrieve it.

External output streams can support all of the usual stream open modifiers. Information about the modifiers is passed to the external output function for handling. (Usually, the external output function delegates the handling of the open modifiers to OmniMark. However, it has the information if other handling is required.)