diff --git a/src/kernel/type_checker.cpp b/src/kernel/type_checker.cpp
index 012ae00de9..b0e6844dca 100644
--- a/src/kernel/type_checker.cpp
+++ b/src/kernel/type_checker.cpp
@@ -981,6 +981,7 @@ lbool type_checker::is_def_eq_offset(expr const & t, expr const & s) {
     return l_undef;
 }
 
+/** \remark t_n, s_n are updated. */
 lbool type_checker::lazy_delta_reduction(expr & t_n, expr & s_n) {
     while (true) {
         lbool r = is_def_eq_offset(t_n, s_n);
@@ -1098,6 +1099,7 @@ bool type_checker::is_def_eq_core(expr const & t, expr const & s) {
     r = is_def_eq_proof_irrel(t_n, s_n);
     if (r != l_undef) return r == l_true;
 
+    /* NB: `lazy_delta_reduction` updates `t_n` and `s_n` even when returning `l_undef`. */
     r = lazy_delta_reduction(t_n, s_n);
     if (r != l_undef) return r == l_true;