<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def external_dependencies(self):
 """ Return all the external images this Dockerfile will depend on These are images from self.dependent_images that aren't defined in this configuration. """

        found = []
        for dep in self.dependent_images:
            if isinstance(dep, six.string_types):
                if dep not in found:
                    yield dep
                    found.append(dep)