Assertion error attempting to convert circuit containing nested conditional to QASM

[cqc-alec]

from pytket.circuit import CircBox, Circuit, Bit, Qubit
from pytket.passes import DecomposeBoxes
from pytket.qasm import circuit_to_qasm_str

cbox = CircBox(Circuit(1, 1).H(Qubit(0), condition=Bit(0)))
c = Circuit(1, 2).add_circbox(cbox, [Qubit(0), Bit(0)], condition=Bit(1))
DecomposeBoxes().apply(c)

print(c.get_commands())
print(circuit_to_qasm_str(c, header="hqslib1"))

Output:

[IF ([c[1]] == 1) THEN IF ([c[0]] == 1) THEN H q[0];]
Traceback (most recent call last):
  File "/home/alec/r/tket/play/1583a.py", line 10, in <module>
    print(circuit_to_qasm_str(c, header="hqslib1"))
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/alec/r/tket/pytket/pytket/qasm/qasm.py", line 1192, in circuit_to_qasm_str
    qasm_writer.add_op(command.op, command.args)
  File "/home/alec/r/tket/pytket/pytket/qasm/qasm.py", line 1953, in add_op
    self.add_conditional(op, args)
  File "/home/alec/r/tket/pytket/pytket/qasm/qasm.py", line 1728, in add_conditional
    assert string is not None
           ^^^^^^^^^^^^^^^^^^
AssertionError

[cqc-alec]

Having checked the specification for QASM 2 (both the original version and the extended version as defined in https://github.com/CQCL/tket/blob/main/pytket/pytket/qasm/grammar.py ), I've realized that nested conditionals are not supported in QASM at all. So the only issue here is that we fail with an AssertionError rather than a more friendly error.