open string-expression with referents-allowed
     defaulting {default value}
     as file string-expression

This modifier allows a programmer to provide a definition (default value) for every referent that's written out. A defaulting declaration can be specified with referents-allowed in an open action. There are two possible forms, either as one string expression or two, and these provide an effective value for referents as follows:

• If there is one string expression, then that is the effective value. To ensure that undefined referents are simply omitted from any stream to which they are written, use the zero-length string as their default effective value.
• If there are two string expressions, then the effective value is the name of the referent, with the first string preceding it and the second following it. To get just the name with no surrounding text, use two zero-length strings.

The defaulting part used to provide the effective value of an undefined referent is the defaulting part associated with the "final destination" of the referent.