def _create_patterns(self, properties=None):
        """
        Return a list (of length patterns_per_label) of
        PatternGenerator instances.  Should use pattern_type and
        pattern_parameters to create each pattern.

        properties is a dictionary, e.g. {'pattern_label':
        pattern_label}, which can be used to create PatternGenerators
        depending on the requested pattern_label
        """
        return [self.pattern_type(**self.pattern_parameters)
                for i in range(self.patterns_per_label)]