Change trajectory in moveit while the robot is moving

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HI, I am using Moveit to plan for robot arm motion to account for base movements. I want to use the trajectory execution manager to overwrite the future trajectory of the robot while it is in motion to quickly adapt to changes, however it seems like the only way to do this is to write a plugin for moveit. How do I write a plugin for moveit so that I can access the trajectory execution manager to use the pushAndExecute function?

I am on Ubuntu 16.04 and ROS Kinetic.

Thanks