When using the curve operations in the animation menu (Predefined/...) or in the curve editor, an expression is applied to the existing keyframe animation in a non destructive way.
I.e.: "Predefined/Negate" simply turns "curve" into "-curve" where
'curve' holds the keyframe information.
To clear any of these operation and return to the original keyframed curve simply add this to your menu.tcl:
menu "Animation/Predefined/Reset" {set_expression {curve}}
.
