docs: def getLeader(self, vehID, dist=0.):
          """getLeader(string, double) -> (string, double)
  
          Return the leading vehicle id together with the distance. The distance
          is measured from the front + minGap to the back of the leader, so it does not include the
          minGap of the vehicle.
          The dist parameter defines the maximum lookahead, 0 calculates a lookahead from the brake gap.
          Note that the returned leader may be farther away than the given dist.
          """
          self._connection._beginMessage(
              tc.CMD_GET_VEHICLE_VARIABLE, tc.VAR_LEADER, vehID, 1 + 8)
          self._connection._string += struct.pack("!Bd", tc.TYPE_DOUBLE, dist)
          return _readLeader(self._connection._checkResult(tc.CMD_GET_VEHICLE_VARIABLE, tc.VAR_LEADER, vehID))