from qiskit import QuantumCircuit

# GhostCompose: qc.compose(sub) is non-mutating in Qiskit, but its return
# value is discarded, so the composed circuit is silently dropped. Engine A
# only.
qc = QuantumCircuit(1, 1)
sub = QuantumCircuit(1, 1)
sub.h(0)
qc.compose(sub)    # BUG: compose() return value discarded (GhostCompose)