parameters: (self, key)
return_statement: return key
docstring: Get successor to key, raises KeyError if key is max key
          or key does not exist.
docstring_summary: Get successor to key, raises KeyError if key is max key
          or key does not exist.
docstring_tokens: Get successor to key raises KeyError if key is max key or key does not exist .
function: def succ_key(self, key):
          """ Get successor to key, raises KeyError if key is max key
          or key does not exist.
          """
          key, value = self.succ_item(key)
          return key
function_tokens: def succ_key ( self , key ) : key , value = self . succ_item ( key ) return key