Issue 257/xp formalism

.pylintrc

@@ -33,7 +33,7 @@ unsafe-load-any-extension=no
 # A comma-separated list of package or module names from where C extensions may
 # be loaded. Extensions are loading into the active Python interpreter and may
 # run arbitrary code
-extension-pkg-whitelist=retworkx, numpy, tweedledum
+extension-pkg-whitelist=rustworkx, numpy, tweedledum
 
 
 [MESSAGES CONTROL]

qiskit_qec/__init__.py

@@ -15,6 +15,7 @@
 from logging import NullHandler
 
 from . import (
+    arithmetic,
     circuits,
     codes,
     decoders,

qiskit_qec/arithmetic/__init__.py

@@ -0,0 +1,13 @@
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2021.
+#
+# This code is licensed under the Apache License, Version 2.0. You may
+# obtain a copy of this license in the LICENSE.txt file in the root directory
+# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
+#
+# Any modifications or derivative works of this code must retain this
+# copyright notice, and modified files need to carry a notice indicating
+# that they have been altered from the originals.
+
+from .modn import gcd_ext, quo, div, ann, stab, unit