mjo/cone/faces.py: use simpler dual face formula

diff --git a/mjo/cone/faces.py b/mjo/cone/faces.py
index 6c5843e860ef9da5151150103223a1d507914fe1..0d9986d23c546af4af8e4a843faca9884b20c649 100644 (file)
--- a/mjo/cone/faces.py
+++ b/mjo/cone/faces.py
@@ -207,9 +207,8 @@ def dual_face(K,F):
         True
 
     """
-    # Ensure that F is actually a face of K before continuing.
     if not F.is_face_of(K):
         raise ValueError("%s is not a face of %s" % (F,K))
 
-    span_F = Cone((c*g for c in [1,-1] for g in F), F.lattice())
-    return K.dual().intersection(span_F.dual())
+    # Hilgert, Hoffman, and Lawson Proposition I.1.9
+    return K.dual().intersection(-F.dual())