fix PsSubscript set_child

src/pystencils/backend/ast/expressions.py

@@ -230,7 +230,7 @@ class PsSubscript(PsLvalue, PsExpression):
         return (self._arr,) + tuple(self._index)
 
     def set_child(self, idx: int, c: PsAstNode):
-        idx = range(len(self._index))[idx]
+        idx = range(len(self._index) + 1)[idx]
         match idx:
             case 0:
                 self.array = failing_cast(PsExpression, c)