fix: Emit nested binary expressions for classical ops

[qartik]

Description

Fixes:

• Error simulating circuit with condition based on more than two bits #215
• Provide email ID for users to be able to reach out #142

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have performed a self-review of my code
• I have added tests that prove my fix is effective or that my feature works
• I have updated the changelog with any user-facing changes

[qartik]

Note endianness of pytket: #162 (comment)

Need to be careful of endianness here. The conditional value in pytket is little-endian, e.g. a value of 2 on the bits [a[0], a[1]] means a[0] == 0 && a[1] == 1.

[cqc-alec]

LGTM, just a suggestion on edge-case handling.

[cqc-alec]

I have a feeling this will error out if len(args) is 0 or 1. While these may be rare edge cases it would be good to handle them or at least raise an informative exception.

[PabloAndresCQ]

This would make sense, but PECOS does not seem to be able to deal with "cop": "&" with fewer than two arguments.

ValueError: not enough values to unpack (expected 2, got 1)

Is there a way to add a "True" constant to an expression in PHIR? The AND with a single element args == [x] could be implemented as (x == val) & (x == val), but in the case of args == [] you'd need to be able to output True somehow.

Not sure if we'd want to do this, though. Is it OK for pytket-phir to create valid PHIR when the conditions the user inputted would not be accepted by PECOS if done directly? It's true that in this case, the action of AND on 1 and 0 arguments is not ambiguous, but it still makes me uneasy. I'd just throw an exception with informative message.

[qartik]

The function is only called at one place when len(args) is not 1, but for generality (and reusability), it's certainly worth doing an edge case check. Implemented and pushed.

[PabloAndresCQ]

Looks good to me too. I've checked that, with this change, the circuit from the project I was working on is converted to PHIR that can be interpreted by PECOS. Thanks!