libsmi error textual-convention-nested

Error text: textual convention `__' can not be derived from the textual convention `__'

Reference: RFC 2579 section 3.5

3.5.  Mapping of the SYNTAX clause

   The SYNTAX clause, which must be present, defines abstract data
   structure corresponding to the textual convention.  The data
   structure must be one of the alternatives defined in the ObjectSyntax
   CHOICE or the BITS construct (see section 7.1 in [2]).  Note that
   this means that the SYNTAX clause of a Textual Convention can not
   refer to a previously defined Textual Convention.