Generates a string of the rdf namespaces listed used in the
            framework

            format: "sparql" or "turtle"

def prefix(self, format="sparql"):
        ''' Generates a string of the rdf namespaces listed used in the
            framework

            format: "sparql" or "turtle"
        '''

        lg = logging.getLogger("%s.%s" % (self.ln, inspect.stack()[0][3]))
        lg.setLevel(self.log_level)

        _return_str = ""
        if format.lower() == "sparql":
            return "\n".join([ns._sparql_ for ns in self.namespaces])
        elif format.lower() in ["turtle", "ttl"]:
            return "\n".join([ns._ttl_ for ns in self.namespaces])
        elif format.lower() in ["rdf", "xml", "rdf/xml"]:
            return "<rdf:RDF %s>" % \
                    " ".join([ns._xml_ for ns in self.namespaces])
        else:
            raise NotImplementedError("'%s' is not a valid prefix type." %
                                      format)