Fix bug of flatten_and_balance for leaves

qctoolkit/hardware/program.py

@@ -112,6 +112,8 @@ def repetition_count(self, val) -> None:
         self._repetition_count = new_repetition
 
     def unroll(self) -> None:
+        if self.is_leaf():
+            raise RuntimeError('Leaves cannot be unrolled')
         for i, e in enumerate(self.parent):
             if id(e) == id(self):
                 self.parent[i:i+1] = (child.copy_tree_structure(new_parent=self.parent)
@@ -253,9 +255,11 @@ def flatten_and_balance(self, depth: int) -> None:
                 sub_program[:] = sub_sub_program[:]
                 sub_program.waveform = sub_sub_program.waveform
 
-            else:
+            elif not sub_program.is_leaf():
                 sub_program.unroll()
 
+            else:
+                i += 1
 
 class ChannelSplit(Exception):
     def __init__(self, channel_sets):