code: def injectables_spec_for_key(self, key):
      
      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]
docs: Given a key, yield a singular spec representing that key.
  
      :API: public