Gear that can interpolate between two input gears

Hi,

I'm looking for a type of gear system that can do the following:

There are three input gears: A, B and control C

There is an output gear O.

If control C is rotated to the left, O has the same rotation as A

If control C is rotated to the right, O has the same rotation as B

If control C is something in-between, the output O is a linear interpolation between input A and input B, so it smoothly goes from the one to the other as C goes to the right.

None of the gears need to be able to make full rotations, just from left to right. So if A is pointed left and B is pointed right, C going from left to right makes the output from left to right. But if A is pointed right and B is pointed left, C going from left to right makes the output go from right to left. And if A and B are equal, rotating C does nothing, the output is equal to both inputs then.

I thought something like a differential gear or planetary gears might be able to do it, but I can't find anything that works. Ideally it'd be as simple as possible.

Any ideas? Thanks!