_mixture_ should be able to return List[Tuple[float, Operation]] instead of List[Tuple[float, matrix]]

[Strilanc]

Where it is required that the operations be unitary.

This is beneficial because it allows you to return mixtures of large operations, without having to generate their unitaries.

Fixing this mostly involves finding usages of mixture and making them account for this.

[dabacon]

You mean Gate instead of Operation?

[Strilanc]

Good point. A Gate's _mixture_ would return gates, whereas an Operation's _mixture_ would return operations. Then, in GateOperation, there would be a conversion.

[c-poole]

Is this a duplicate of #1626? Also, I can give this a shot.

[Strilanc]

Yup, it's a duplicate. Both by me. Four days apart. Wow.