code: def override (overrider_id, overridee_id):
      
      assert isinstance(overrider_id, basestring)
      assert isinstance(overridee_id, basestring)
  
      __overrides.setdefault(overrider_id, []).append(overridee_id)
docs: Make generator 'overrider-id' be preferred to
      'overridee-id'. If, when searching for generators
      that could produce a target of certain type,
      both those generators are amoung viable generators,
      the overridden generator is immediately discarded.
  
      The overridden generators are discarded immediately
      after computing the list of viable generators, before
      running any of them.