code: func (c *ConfigLookup) Lookup(key string) (interface{}, bool) {
  	//loop through each backend to find the value by key, fallback to next one if not found
  	for _, backend := range c.backends {
  		if backend == nil {
  			continue
  		}
  		val, ok := backend.Lookup(key)
  		if ok {
  			return val, true
  		}
  	}
  	return nil, false
  }
docs: //Lookup returns value for given key