code: def _acquire_connection(self, host, port, use_ssl=False, tunnel=True):
          '''Return a connection.'''
          if hasattr(self._connection_pool, 'acquire_proxy'):
              connection = yield from \
                  self._connection_pool.acquire_proxy(host, port, use_ssl,
                                                      tunnel=tunnel)
          else:
              connection = yield from \
                  self._connection_pool.acquire(host, port, use_ssl)
  
          self._connections.add(connection)
  
          return connectiof _acquire_connection(self, host, port, use_ssl=False, tunnel=True):
          '''Return a connection.'''
          if hasattr(self._connection_pool, 'acquire_proxy'):
              connection = yield from \
                  self._connection_pool.acquire_proxy(host, port, use_ssl,
                                                      tunnel=tunnel)
          else:
              connection = yield from \
                  self._connection_pool.acquire(host, port, use_ssl)
  
          self._connections.add(connection)
  
          return connection
docs: Return a connection.