From a8ab023e2470e29737d1ad3fd50bb2d154a4ee81 Mon Sep 17 00:00:00 2001
From: Steve Bronder <sbronder@flatironinstitute.org>
Date: Mon, 27 Nov 2023 18:02:07 -0500
Subject: [PATCH] update docs

---
 stan/math/rev/core/arena_matrix.hpp | 5 +++++
 stan/math/rev/core/var.hpp          | 2 +-
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/stan/math/rev/core/arena_matrix.hpp b/stan/math/rev/core/arena_matrix.hpp
index b65bed948cd..a0b24725fb7 100644
--- a/stan/math/rev/core/arena_matrix.hpp
+++ b/stan/math/rev/core/arena_matrix.hpp
@@ -128,6 +128,11 @@ class arena_matrix : public Eigen::Map<MatrixType> {
     Base::operator=(a);
     return *this;
   }
+  /**
+   * Forces hard copying matrices into an arena matrix 
+   * @tparam T Any type assignable to `Base`
+   * @param x the values to write to `this`
+   */
   template <typename T>
   void hard_copy(const T& x) {
     Base::operator=(x);
diff --git a/stan/math/rev/core/var.hpp b/stan/math/rev/core/var.hpp
index f02fbaec05c..5553296383e 100644
--- a/stan/math/rev/core/var.hpp
+++ b/stan/math/rev/core/var.hpp
@@ -390,7 +390,7 @@ class var_value<T, internal::require_matrix_var_value<T>> {
     reverse_pass_callback(
         [this_vi = this->vi_, other_vi = other.vi_]() mutable {
           other_vi->adj_ += this_vi->adj_;
-          //
+          // Reset the adjoint for `this` to replicate SoA before assignment
           this_vi->adj_.setZero();
         });
   }