added some comments within qc_to_dag to explain the qc_index attribute

src/qrisp/core/compilation.py

@@ -526,6 +526,8 @@ def reorder_qc(qc):
     # sub_sort = lambda G : topological_sort(G, prefer = mcx_identifier, delay = nmcx_identifier)
     # for n in topological_sort(G, prefer = dealloc_identifier, delay = alloc_identifer, sub_sort = sub_sort):
 
+    # The sub_sort function takes the indices of the gates in the source qc as
+    # a sorting index. This induces a valid topological ordering because of [proof]
     def sub_sort(dag):
         nodes = list(dag.nodes())
         nodes.sort(key = lambda x : x.qc_index)

src/qrisp/uncomputation/unqomp.py

@@ -124,6 +124,13 @@ def dag_from_qc(qc, remove_init_nodes=False):
         #   + str(node_counter[instr.qubits[-1]]), instr)
         node = UnqompNode(str(node_counter[instr.qubits[-1]]), instr)
 
+        # We add the index of the corresponding gate to the node object,
+        # because this information can be used for stable topological ordering:
+        # The allocation algorithm performs a topological order based on the ancestors
+        # of a certain nodes. The algorithm can subsequently use another topological
+        # ordering to sort the ancestors. To make the sorting algorithm "stable" ie.
+        # it preserves the previous order where possible, we use the indices of the source
+        # circuit as sorting index.
         node.qc_index = i
 
         if instr.op.name == "qb_dealloc":