bugfix in eval/substitution operator

1 files changed, 3 insertions, 3 deletions

diff --git a/polymatrix/expression/mixins/derivativeexprmixin.py b/polymatrix/expression/mixins/derivativeexprmixin.py
index 5fce215..183608b 100644
--- a/polymatrix/expression/mixins/derivativeexprmixin.py
+++ b/polymatrix/expression/mixins/derivativeexprmixin.py
@@ -7,7 +7,7 @@ from polymatrix.expression.mixins.expressionbasemixin import ExpressionBaseMixin
 from polymatrix.polymatrix.polymatrix import PolyMatrix
 from polymatrix.expressionstate.expressionstate import ExpressionState
 from polymatrix.expression.utils.getderivativemonomials import get_derivative_monomials
-from polymatrix.expression.utils.getvariableindices import get_variable_indices
+from polymatrix.expression.utils.getvariableindices import get_variable_indices_from_variable
 
 
 class DerivativeExprMixin(ExpressionBaseMixin):
@@ -18,7 +18,7 @@ class DerivativeExprMixin(ExpressionBaseMixin):
 
     @property
     @abc.abstractmethod
-    def variables(self) -> typing.Union[tuple, ExpressionBaseMixin]:
+    def variables(self) -> ExpressionBaseMixin:
         ...
 
     @property
@@ -33,7 +33,7 @@ class DerivativeExprMixin(ExpressionBaseMixin):
     ) -> tuple[ExpressionState, PolyMatrix]:
 
         state, underlying = self.underlying.apply(state=state)        
-        state, diff_wrt_variables = get_variable_indices(state, self.variables)
+        state, diff_wrt_variables = get_variable_indices_from_variable(state, self.variables)
 
         assert underlying.shape[1] == 1, f'{underlying.shape=}'