the difference between a frame and a keyframe is that if one deletes a keyframe,
then the interpolation around that point changes, but when one deletes a range of
frames, they're completely gone, and now the animation is shorter by that
number of frames.  deleting keyframes has no effect on the length of the animation.
the number of frames determines the length of the animation, but the keyframes
only determine how the frames are interpolated.

o número dos "frames" determina o comprimento do animation, mas os "keyframes"
determinam somente como os frames são calculados.