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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 create_main_target (self, name):
 """ Returns a 'MainTarget' class instance corresponding to the 'name'. """

        assert isinstance(name, basestring)
        if not self.built_main_targets_:
            self.build_main_targets ()

        return self.main_targets_.get (name, None)