fix BQSKit#302

bqskit/ir/lang/qasm2/qasm2.py

@@ -7,6 +7,7 @@
 from bqskit.ir.lang.language import Language
 from bqskit.ir.lang.qasm2.parser import parse
 from bqskit.ir.lang.qasm2.visitor import OPENQASMVisitor
+from bqskit.ir.gates.measure import MeasurementPlaceholder
 
 if TYPE_CHECKING:
     from bqskit.ir.circuit import Circuit
@@ -22,7 +23,20 @@ def encode(self, circuit: Circuit) -> str:
 
         source = "OPENQASM 2.0;\ninclude \"qelib1.inc\";\n"
         source += f'qreg q[{circuit.num_qudits}];\n'
+        is_classical_register_present = False
         for gate in circuit.gate_set:
+            # add at maximum one classical register definition per gate
+            if isinstance(gate, MeasurementPlaceholder):
+                if is_classical_register_present:
+                    # skip all subsequent classical register definitions
+                    continue
+
+            if isinstance(gate, MeasurementPlaceholder):
+                is_classical_register_present = True
+
+            print(gate)
+            print("Gate definition:")
+            print(gate.get_qasm_gate_def())
             source += gate.get_qasm_gate_def()
 
         for op in circuit: