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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def add_dependency (self, targets, sources):
 """Adds a dependency from 'targets' to 'sources' Both 'targets' and 'sources' can be either list of target names, or a single target name. """

        if isinstance (targets, str):
            targets = [targets]
        if isinstance (sources, str):
            sources = [sources]
        assert is_iterable(targets)
        assert is_iterable(sources)

        for target in targets:
            for source in sources:
                self.do_add_dependency (target, source)