Displaying a precomputed trajectory problem

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When I follow the instructions on this page to plot trajectories in Rviz previously recorded from a .YAML file I get the following error: (add all sesion)

diego@ubuntu:~$ rosrun walk_msgs generator _yaml:=/home/diego/Descargas/dropping_the_ball.yaml
fatal error: yaml-cpp: error at line 7, column 3: key not found: left-hand
 [ERROR] [1362606276.617362117]: fatal error: yaml-cpp: error at line 7, column 3: key not found:     left-hand***

And don't show anything. Any sugestion?

I work on ROS FUERTE