code: def ldContains(self, what, key):
      
      if isListKey(key):
        i = listKeyIndex(key)
        return i < len(what) and what[i] != missing_value
      else:
        return key in what and what[key] != missing_value
docs: List/dictinary/missing-aware contains.
  
      If the value is a "missing_value", we'll treat it as non-existent
      so it will be overwritten by an empty list/dict when necessary to
      assign child keys.