n00b at JSFL. Currently making my first script to automate some tasks in Adobe Animate. One of the steps is keeping my selected layers' first keyframe, while clearing out the rest that follow after. How would I go about asking the computer to do this?

Screenshot "A" is what I'd like to achieve, screenshot "B" is what I get when I run:

an.getDocumentDOM().getTimeline().clearKeyframes();

How would I break this down further? (super beginner, haha!)