diff --git a/functions/core_math_accuracy_test.cpp b/functions/core_math_accuracy_test.cpp
index 599c3f45df..ab7a9c4f98 100644
--- a/functions/core_math_accuracy_test.cpp
+++ b/functions/core_math_accuracy_test.cpp
@@ -4,6 +4,7 @@
 
 #include "core-math/cos.h"
 #include "core-math/sin.h"
+#include "functions/multiprecision.hpp"
 #include "glog/logging.h"
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
@@ -13,9 +14,10 @@
 
 namespace principia {
 namespace functions {
-namespace _multiprecision {
+namespace _core_math_accuracy_test {
 
 using namespace boost::multiprecision;
+using namespace principia::functions::_multiprecision;
 using namespace principia::testing_utilities::_approximate_quantity;
 using namespace principia::testing_utilities::_is_near;
 
diff --git a/functions/sin_cos_test.cpp b/functions/sin_cos_test.cpp
index 1acdc2c77c..c888e1d60a 100644
--- a/functions/sin_cos_test.cpp
+++ b/functions/sin_cos_test.cpp
@@ -9,18 +9,28 @@
 #include "glog/logging.h"
 #include "gtest/gtest.h"
 #include "numerics/next.hpp"
-#include "quantities/numbers.hpp"
+#include "testing_utilities/almost_equals.hpp"
+#include "numerics/sin_cos.hpp"
+#include "quantities/numbers.hpp"  // 🧙 For π.
 #include "testing_utilities/almost_equals.hpp"
 
 // This test lives in `functions` to avoid pulling `boost` into `numerics`.
+// It uses neither the `functions` nor the `numerics` namespace so that the Sin
+// and Cos from both (`principia::functions::_multiprecision` and
+// `principia::numerics::_sin_cos`) are made visible by the using directives
+// below.
+
 namespace principia {
-namespace numerics {
-namespace _sin_cos {
+namespace numerics_multiprecision_tests {
 
 using namespace boost::multiprecision;
+using namespace principia::functions::_multiprecision;
 using namespace principia::numerics::_next;
+using namespace principia::numerics::_sin_cos;
 using namespace principia::testing_utilities::_almost_equals;
 
+namespace {
+
 class SinCosTest : public ::testing::Test {};
 
 TEST_F(SinCosTest, Random) {
@@ -44,8 +54,7 @@ TEST_F(SinCosTest, Random) {
     double const principia_argument = uniformly_at(random);
     auto const boost_argument = cpp_rational(principia_argument);
     {
-      auto const boost_sin =
-          functions::_multiprecision::Sin(boost_argument);
+      auto const boost_sin = Sin(boost_argument);
       double const principia_sin = Sin(principia_argument);
       auto const sin_error =
           abs(boost_sin - static_cast<cpp_bin_float_50>(principia_sin));
@@ -61,8 +70,7 @@ TEST_F(SinCosTest, Random) {
       }
     }
     {
-      auto const boost_cos =
-          functions::_multiprecision::Cos(boost_argument);
+      auto const boost_cos = Cos(boost_argument);
       double const principia_cos = Cos(principia_argument);
       auto const cos_error =
           abs(boost_cos - static_cast<cpp_bin_float_50>(principia_cos));
@@ -143,6 +151,6 @@ TEST_F(SinCosTest, HardReduction) {
               AlmostEquals(-4.687165924254627611122582801963884e-19, 0));
 }
 
-}  // namespace _sin_cos
-}  // namespace numerics
+}  // namespace
+}  // namespace numerics_multiprecision_tests
 }  // namespace principia
diff --git a/geometry/serialization_body.hpp b/geometry/serialization_body.hpp
index feea4e0b95..9236f988f1 100644
--- a/geometry/serialization_body.hpp
+++ b/geometry/serialization_body.hpp
@@ -6,6 +6,7 @@
 #include "geometry/grassmann.hpp"
 #include "geometry/point.hpp"
 #include "quantities/quantities.hpp"
+#include "quantities/serialization.hpp"
 
 namespace principia {
 namespace geometry {
diff --git a/sourcerer/include_what_you_using.cs b/sourcerer/include_what_you_using.cs
index 27aab28820..3fdb6793e0 100644
--- a/sourcerer/include_what_you_using.cs
+++ b/sourcerer/include_what_you_using.cs
@@ -186,7 +186,10 @@ private static void FixIncludes(Parser.File file,
     foreach (string ns in using_namespaces) {
       var segments = ns.Split("::", StringSplitOptions.RemoveEmptyEntries);
       var include_path = Array.Empty<string>();
-      bool skip = false;
+      if (ns == file.file_namespace_full_name) {
+        // Don't add an include for our own header.  This matters for tests.
+        continue;
+      }
       foreach (string segment in segments) {
         if (segment == "principia") {
           continue;
@@ -195,24 +198,17 @@ private static void FixIncludes(Parser.File file,
                    segment == "std") {
           // We have using directives for namespaces in absl or std, don't emit
           // an include for them.
-          skip = true;
-          continue;
+          goto next_using_directive;
         } else if (segment[0] == '_') {
-          // Don't add an include for our own header.  This matters for tests.
-          if (segment == file.file_namespace_simple_name) {
-            skip = true;
-          } else {
-            string header_filename = Regex.Replace(segment, @"^_", "");
-            include_path = include_path.Append(header_filename).ToArray();
-          }
+          string header_filename = Regex.Replace(segment, @"^_", "");
+          include_path = include_path.Append(header_filename).ToArray();
           break;
         } else {
           include_path = include_path.Append(segment).ToArray();
         }
       }
-      if (!skip) {
-        new_include_paths.Add(include_path);
-      }
+      new_include_paths.Add(include_path);
+      next_using_directive: {}
     }
 
     // Extract the includes.