Classical operations composed of several binary operations incorrectly converted to PHIR

[cqc-alec]

from pytket.circuit import Circuit
from pytket.phir.api import pytket_to_phir

circ = Circuit(4)
a = circ.add_c_register("a", 4)
b = circ.add_c_register("b", 1)
circ.add_classicalexpbox_bit(a[0] ^ a[1] ^ a[2] ^ a[3], [b[0]])

print(pytket_to_phir(circ))

Output:

{
  "format": "PHIR/JSON",
  "version": "0.1.0",
  "metadata": {
    "source": "pytket-phir v0.4.3"
  },
  "ops": [
    {
      "data": "cvar_define",
      "data_type": "u32",
      "variable": "b",
      "size": 1
    },
    {
      "data": "cvar_define",
      "data_type": "u32",
      "variable": "a",
      "size": 4
    },
    {
      "//": "ClassicalExpBox a[0], a[1], a[2], a[3], b[0];"
    },
    {
      "cop": "=",
      "returns": [
        [
          "b",
          0
        ]
      ],
      "args": [
        {
          "cop": "^",
          "args": [
            {
              "cop": "^",
              "args": [
                {
                  "cop": "^",
                  "args": [
                    "a",
                    "a"
                  ]
                },
                "a"
              ]
            },
            [
              "a",
              3
            ]
          ]
        }
      ]
    }
  ]
}

Note that {"cop": "^", "args": [{"cop": "^", "args": ["a", "a"]}, "a"]} includes the whole register a in its arguments three times, instead of the bits a[0], a[1] and a[2].

This leads to incorrect simulation results.

Reproduced with pytket-phir 0.4.3, pytket 1.25.0.