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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 injectables_spec_for_key(self, key):
 """Given a key, yield a singular spec representing that key. :API: public """

    specs = self.injectables_specs_for_key(key)
    specs_len = len(specs)
    if specs_len == 0:
      return None
    if specs_len != 1:
      raise self.TooManySpecsForKey('injectables spec mapping for key included {} elements, '
                                    'expected 1'.format(specs_len))
    return specs[0]