Chunk.then_reflow calls Flow.with_edits with non-existent arg "obs_name"

[JLenssen]

Issue description

I was playing around with chunks that map logical X to logical Z and noticed that then_reflow uses old Flow API when calling with_edits.

import stimflow as sf
import stim

lz = sf.PauliMap.from_zs([0]).with_obs_name('LZ')
lx = sf.PauliMap.from_xs([0]).with_obs_name('LX')
lz_aliased = lx.with_obs_name('LZ')  # same obs_name as lz, so Flow validator passes

circuit = stim.Circuit('''
   QUBIT_COORDS(0, 0) 0
   H 0
''')

chunk = sf.Chunk(
   circuit=circuit,
   flows=[sf.Flow(start=lz, end=lz_aliased)],
)

reflow = sf.ChunkReflow(out2in={lx: [lz_aliased]})

# _then_reflow calls flow.with_edits(obs_name=None) with has no obs_name
try:
   result = chunk.then(reflow)
except TypeError as e:
   print('Bug:', e)

Additional question

Could you also clarify the use of obs_name?

A flow's start and end PauliMap need to have matching names

        if isinstance(start, PauliMap) and isinstance(end, PauliMap) and start.obs_name != end.obs_name:
            raise ValueError(f'{start.obs_name=} != {end.obs_name=}')

The examples in the repo use "LX" and "LZ". Following the example I have a hard time implementing a chunk with flows that maps "LX" to "LZ". I feel like an observable index plus basis indicator, e.g. (0, "LX") would work but the unit tests for C4 code use "LX0" and "LX1" so I tried to stick with the convention.

A short explanation or code snippet would help a lot to correct my mental model. Thanks a lot!

[Strilanc]

Currently, the start and end of a flow having different names isn't something that is supported. As you've noticed, it used to literally be the flow that was named rather than the start/end and so it used to not even be stateable in principle. It actually hadn't occurred to me to allow the start name to differ from the end name... but this is actually a really good idea. It's a way of decoupling chunks from each other.

I'll keep this mind, and try to incorporate it alongside other needed changes (particularly to ChunkReflow).

As for the specific bug you noticed, I will fix it by actually adding support for flow.with_edits(obs_name=...) in the form of renaming both the start and the end (which is otherwise tedious).

[JLenssen]

Thanks @Strilanc for the quick reply and the fixes!