Dax Fohl

xref #3678 #4193

tbh I'd consider removing immutability from the next version of Cirq. Immutability is nice for lots of things, but I don't feel like it's providing all that much value here....

Is the transformer library not already handling these? It looks from the code like it should be working around tags on CircuitOperations here. https://github.com/quantumlib/Cirq/blob/614c78ae14dfa063d56023fadd8040415e89986b/cirq-core/cirq/transformers/transformer_primitives.py#L165

I can't for the life of me remember how to set up the dev environment, but looking through the code, it looks like you _might_ be able to work around...

I closed the linked PR with the fix because I think the right thing to do here is to do the renormalization in `partial_trace` itself. The `einsum` there is losing...

I don't know if it's the _right_ way to do it (I've been working with meteor for only a couple days and am far from understanding the details, and I've...

Is it worth considering adding I as a proper Pauli gate? The way they silently get removed when working with Pauli strings has come up before, and the asymmetry can...

`PauliString._circuit_diagram_info_` gets around this by returning `NotImplemented` [if it is empty](https://github.com/quantumlib/Cirq/blob/master/cirq-core/cirq/ops/pauli_string.py#L366-L367), which causes the diagramming logic to put the op below the circuit along with global phase and other non-qubit...

I'd like to do this, but whatever we do I'd like to make it compatible for use in the simulator framework. What "level" are you most interested in here? 1....

FWIW my preference is (2) above. I think (1) is great but probably higher level than what's useful from a QIS standpoint. (3) is nice in that it avoids bringing...