Fix MonomialIndex ordering and MonomialIndex.is_constant

1 files changed, 7 insertions, 4 deletions

diff --git a/polymatrix/polymatrix/index.py b/polymatrix/polymatrix/index.py
index 79b8161..384feb8 100644
--- a/polymatrix/polymatrix/index.py
+++ b/polymatrix/polymatrix/index.py
@@ -1,7 +1,8 @@
 from __future__ import annotations
+
 from typing import NamedTuple, Iterable, cast
 from collections import UserDict
-from itertools import filterfalse
+from itertools import dropwhile
 
 # TODO: remove these types, they are here for backward compatiblity
 MonomialData = tuple[tuple[int, int], ...]
@@ -56,9 +57,11 @@ class MonomialIndex(tuple[VariableIndex]):
         # they have the same degree the index counts
         if self.degree == other.degree:
             # Assumes that monomialindex is sorted!
-            # FIXME: if the first variable is also the same continue until one
+            # if the first variable is also the same continue until one
             # finishes or the values differ.
-            return self[0] < other[0]
+            # FIXME: check that return value is not none
+            s, o = next(dropwhile(lambda t: t[0] == t[1], zip(self, other)))
+            return s < o
 
         return self.degree < other.degree
 
@@ -75,7 +78,7 @@ class MonomialIndex(tuple[VariableIndex]):
     @staticmethod
     def is_constant(index: MonomialIndex) -> bool:
         """ Returns true if it is indexing a constant monomial. """
-        return len(index) > 1
+        return len(index) == 0
 
     @staticmethod
     def sort(index: MonomialIndex) -> MonomialIndex: