diff --git a/sxt/algorithm/base/mapper.h b/sxt/algorithm/base/mapper.h
index ae893d4b..1a838323 100644
--- a/sxt/algorithm/base/mapper.h
+++ b/sxt/algorithm/base/mapper.h
@@ -28,7 +28,7 @@ namespace sxt::algb {
  * Mapper turns an index into a value.
  */
 template <class M>
-concept mapper = requires(M m, typename M::value_type& x, unsigned int i, void* data) {
+concept mapper = requires(M m, typename M::value_type& x, unsigned int i) {
   { m.map_index(i) } noexcept -> std::convertible_to<typename M::value_type>;
   { m.map_index(x, i) } noexcept;
 };
diff --git a/sxt/algorithm/base/reducer.h b/sxt/algorithm/base/reducer.h
index 27f7b704..42cea3cc 100644
--- a/sxt/algorithm/base/reducer.h
+++ b/sxt/algorithm/base/reducer.h
@@ -25,9 +25,7 @@ namespace sxt::algb {
  * See https://developer.download.nvidia.com/assets/cuda/files/reduction.pdf
  */
 template <class R>
-concept reducer = requires(typename R::value_type& x, typename R::value_type& xv,
-                           typename R::value_type& yv, typename R::value_type& z) {
-  { R::accumulate_inplace(x, z) } noexcept;
-  { R::accumulate_inplace(xv, yv) } noexcept;
+concept reducer = requires(typename R::value_type& x) {
+  { R::accumulate_inplace(x, x) } noexcept;
 };
 } // namespace sxt::algb