Adobe 12040118 - After Effects Standard Tutorial page 191

very simple expression to the Opacity property to set it to 100%: 100.
Click the Enable Expression button to toggle the expression on and off, which toggles the keyframes off and on as a side effect.
If you accidentally delete keyframes, choose Edit > Undo.
Jeff Almasol provides a script on his redefinery website that automatically removes keyframes based on specified criteria—for example, all
keyframes in the work area, all odd-numbered keyframes.
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