diff --git a/doc/concept.md b/doc/concept.md
index f7ec20c..a525a68 100644
--- a/doc/concept.md
+++ b/doc/concept.md
@@ -85,13 +85,15 @@ Therefore, we need to let the model generator know how many arguments / symbolic
 
 ### Constraining viable ranks of different input tensors
 
-Operators like `Concat` require input tensors to have the same ranks. Therefore, we need to somehow constraint the input ranks for `self.inp_ranks` as by default ranks in `self.inp_ranks` are *independent*.
+Operators like `Concat` require input tensors to have the same shape (and thus ranks).
 
-To do so, we set `self.same_inp_dims = True` in initializer:
+To do so, we overload the `same_inp_dims` class variable to `True`:
 
 ```python
-def __init__(...):
-    super().__init__()
+class Concat(AbsOpBase)
     ...
-    self.same_inp_dims = True  # But this is not True for Pool2D and many binary operators.
+    same_inp_dims = True
+    ...
+    def __init__(...):
+        ...
 ```
diff --git a/nnsmith/abstract/op.py b/nnsmith/abstract/op.py
index a07591e..fe47f60 100644
--- a/nnsmith/abstract/op.py
+++ b/nnsmith/abstract/op.py
@@ -257,6 +257,8 @@ def broadcast_to_cons(*shapes: List[Union[z3.ExprRef, int]]) -> List[z3.BoolRef]
 class AbsOpBase(ABC):
     # number of parameters; None means it's fixed that can be inferred through `signature`.
     num_var_param = None
+    # Require the input dimension sizes to be equivalent.
+    same_inp_dims = False
     # whether this op is broadcastable or not
     bcastable = False
     # input dtypes: enumerates all possible input dtype combinations. Size of the list is the number of combinations.
@@ -281,8 +283,6 @@ def __init__(self):
         # NOTE: the concrete values of out_ranks are not useful. Just make sure the length is correct.
         # NOTE: the output shape of input dimensions should be concretized during the execution.
         self.out_ranks = []
-        # Require the input dimension sizes to be equivalent.
-        self.same_inp_dims = False
         # NOTE: the input of operator constructors are all Union[int, z3.ExprRef].
         self.extra_attrs = {}
 
@@ -374,7 +374,6 @@ def concretize_op(op: AbsOpBase, model: Optional[z3.ModelRef]) -> AbsOpBase:
     concrete_op = type(op)(*values)
     concrete_op.inp_ranks = op.inp_ranks
     concrete_op.out_ranks = op.out_ranks
-    concrete_op.same_inp_dims = op.same_inp_dims
     concrete_op.extra_attrs = op.extra_attrs
 
     return concrete_op
@@ -439,7 +438,6 @@ class BcastBinaryOp(BinaryOpBase):
     def __init__(self):
         super().__init__()
         self.inp_ranks = [int_all(), int_all()]
-        self.same_inp_dims = False
         self.bcastable = True
 
     def type_transfer(self, input_shapes: List[AbsTensor]) -> List[AbsTensor]:
@@ -501,7 +499,6 @@ class Where(TernaryOpBase):
     def __init__(self):
         super().__init__()
         self.inp_ranks = [int_all(), int_all(), int_all()]
-        self.same_inp_dims = False
         self.same_inp_dtypes = True
         self.bcastable = True
 
@@ -1859,6 +1856,7 @@ class Concat(AbsOpBase):
     MAX_ARITY = 5
     MAX_RANK = 5
     out_dtypes = [(i,) for i in DTYPE_ALL]
+    same_inp_dims = True
 
     def __str__(self) -> str:
         return "Concat " + str(self.extra_attrs).replace(":", "=")
@@ -1869,7 +1867,6 @@ def __init__(self, arity):
         self.arity = arity
         self.inp_ranks = [(int_from(1))] * arity
         self.out_ranks = [(int_from(1))]
-        self.same_inp_dims = True
 
     def _init_concat_axis(self, input_shapes: List[AbsTensor]) -> int:
         if "axis" not in self.extra_attrs: