Error: lidar is not defined in SDF, ignoring lidar

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Hello. I'm new to Gazebo and I'm trying to create a 2D-lidar sensor and connect in later on with ROS. Following the tutorial [http://gazebosim.org/tutorials?cat=gu...] I created the following world file, but instead of ray element I use lidar.

When I run the world file I get the error:

Warning [parser.cc:778] XML Element[lidar], child of element[sensor] not defined in SDF. Ignoring[lidar]. You may have an incorrect SDF file, or an sdformat version that doesn't support this element.

When I change the above file to use ray element it runs smoothly. I follow the SDF format from here: link text so I don't know where the problem might be. Can anyone tell me what I'm missing here?

I use Ubuntu 16.04 and the pre-installed Gazebo 7 and i would prefer to stay with this version. Thanks in advance.