Add sage code

lib/edge_for_dual_edge.py

@@ -0,0 +1,12 @@
+"""Utilities for finding primal edges corresponding to dual edges."""
+from typing import Any, cast
+
+def get_edge_for_dual_edge(dual_edge: Any) -> tuple[int, int, None]:
+    """Return the primal edge shared by both faces of the given dual edge."""
+    edges: list[set[Any]] = []
+    for e in (dual_edge[0] + dual_edge[1]):
+        edge = set(e)
+        if edge in edges:
+            return cast(tuple[int, int, None], e)
+        edges.append(edge)
+    raise ValueError(f"Error finding edge for {dual_edge}")