An animation modifies a value over time.

The primitive animations (null, hold[fi], linear[fi] and bezierf) modify a value over the course of 1 unit of time.

There are three ways to modify an animation:
- Modify the rate that time passes within an animation (identity, sinusoid, exponent, reverse).
- Modify the duration of an animation (scale)
- Combine animations (sequence, parallel)

Derived values (derive[fi]) are attached to animations (attach) and updated as the animation progresses.
For example, you might animation `theta' for a rotating object and then derive `x' and `y' from theta.