handle

    OpenVMS usage:identifier
    type:         quadword (unsigned)
    access:       read only
    mechanism:    by reference
    Identification of the AST request to be cleared. The handle
    argument uniquely identifies the request and is returned when
    the $SET_SYSTEM_EVENT service is called. The handle argument may
    be omitted by specifying a zero address.

 acmode

    OpenVMS usage:access_mode
    type:         longword (unsigned)
    access:       read only
    mechanism:    by value
    Access mode of the system event to be cleared. The acmode
    argument is a longword containing the access mode. The value of
    the access mode is maximized with the access mode of the caller.

 event

    OpenVMS usage:event_code
    type:         longword (unsigned)
    access:       read only
    mechanism:    by value
    The event argument is a value indicating the type of system event
    to be cleared. SYSEVT$C_ALL_EVENTS may be specified to clear all
    event types.