expunge is_commutative from plural

Author: fchapoton

src/sage/rings/polynomial/plural.pyx

@@ -99,6 +99,17 @@ TESTS::
     sage: TestSuite(P).run()
     sage: loads(dumps(P)) is P
     True
+
+    sage: A.<x,y,z> = FreeAlgebra(QQ, 3)
+    sage: P = A.g_algebra(relations={y*x:-x*y}, order = 'lex')
+    sage: P.is_commutative()
+    False
+
+    sage: R.<x,y,z> = FreeAlgebra(QQ, 3)
+    sage: P = R.g_algebra(relations={},order='lex')
+    sage: P.is_commutative()
+    True
+
 """
 from cysignals.memory cimport sig_malloc, sig_free
 
@@ -676,22 +687,7 @@ cdef class NCPolynomialRing_plural(Ring):
         """
         return self._term_order
 
-    def is_commutative(self):
-        """
-        Return ``False``.
-
-        .. TODO:: Provide a mathematically correct answer.
-
-        EXAMPLES::
-
-            sage: A.<x,y,z> = FreeAlgebra(QQ, 3)
-            sage: P = A.g_algebra(relations={y*x:-x*y}, order = 'lex')
-            sage: P.is_commutative()
-            False
-        """
-        return False
-
-    def is_field(self, *args, **kwargs):
+    def is_field(self, *args, **kwargs) -> bool:
         """
         Return ``False``.