This attribute is implemented only in OpenVMS versions of GNAT. Applied to
the name of an entry, it yields a value of the predefined type AST_Handler
(declared in the predefined package System, as extended by the use of
pragma Extend_System (Aux_DEC)). This value enables the given entry to
be called when an AST occurs. For further details, refer to the DEC Ada
Language Reference Manual, section 9.12a.