1 files changed, 12 insertions, 1 deletions
diff --git a/mdpoly/abc.py b/mdpoly/abc.py
index 0a2e783..6637db5 100644
--- a/mdpoly/abc.py
+++ b/mdpoly/abc.py
@@ -4,10 +4,17 @@ from .index import Number, Shape, MatrixIndex, PolyIndex
 from .constants import NUMERICS_EPS
 from .util import iszero
 
-from typing import Self, Iterable, Sequence, Protocol 
+from typing import Self, Iterable, Sequence
+from enum import Enum, auto
 from abc import ABC, abstractmethod
 
 
+class Algebra(Enum):
+    """ Types of algebras. """
+    poly_ring = auto()
+    matrix_ring = auto()
+
+
 class Leaf(ABC):
     """ Leaf of the binary tree. """
     name: str
@@ -27,6 +34,10 @@ class Expr(ABC):
 
     @property
     @abstractmethod
+    def algebra(self) -> Algebra: ...
+
+    @property
+    @abstractmethod
     def shape(self) -> Shape:
         """ Computes the shape of the expression. This method is also used to
         check if the operation described by *Expr* can be perfomed with *left*