diff --git a/include/sampleflow/auxiliary_data.h b/include/sampleflow/auxiliary_data.h
index 11cc67c..45fdb57 100644
--- a/include/sampleflow/auxiliary_data.h
+++ b/include/sampleflow/auxiliary_data.h
@@ -20,6 +20,7 @@
 
 #include <any>
 #include <map>
+#include <string>
 
 
 namespace SampleFlow
diff --git a/tests/condition_01.cc b/tests/condition_01.cc
index af5f81b..86352d9 100644
--- a/tests/condition_01.cc
+++ b/tests/condition_01.cc
@@ -18,13 +18,14 @@
 // the aux_data.
 
 
-#include <iostream>
-
 #include <sampleflow/producers/range.h>
 #include <sampleflow/filters/condition.h>
 #include <sampleflow/consumers/stream_output.h>
 #include <sampleflow/connections.h>
 
+#include <iostream>
+#include <ranges>
+
 
 int main ()
 {
diff --git a/tests/condition_02.cc b/tests/condition_02.cc
index 702334f..85c23e6 100644
--- a/tests/condition_02.cc
+++ b/tests/condition_02.cc
@@ -21,14 +21,15 @@
 // Eratosthenes for computing primes.
 
 
-#include <iostream>
-#include <functional>
-
 #include <sampleflow/producers/range.h>
 #include <sampleflow/filters/condition.h>
 #include <sampleflow/consumers/stream_output.h>
 #include <sampleflow/connections.h>
 
+#include <iostream>
+#include <functional>
+#include <ranges>
+
 
 int main ()
 {
diff --git a/tests/connections_01.cc b/tests/connections_01.cc
index 23074f8..ffa9741 100644
--- a/tests/connections_01.cc
+++ b/tests/connections_01.cc
@@ -21,15 +21,16 @@
 // that setting parentheses works as expected.
 
 
-#include <iostream>
-#include <functional>
-
 #include <sampleflow/producers/range.h>
 #include <sampleflow/filters/condition.h>
 #include <sampleflow/consumers/stream_output.h>
 #include <sampleflow/connections.h>
 
 #include <sstream>
+#include <iostream>
+#include <functional>
+#include <ranges>
+
 
 int main ()
 {
diff --git a/tests/connections_02.cc b/tests/connections_02.cc
index b362437..77d654a 100644
--- a/tests/connections_02.cc
+++ b/tests/connections_02.cc
@@ -25,15 +25,16 @@
 // use temporary objects (created in-place) for these.
 
 
-#include <iostream>
-#include <functional>
-
 #include <sampleflow/producers/range.h>
 #include <sampleflow/filters/condition.h>
 #include <sampleflow/consumers/stream_output.h>
 #include <sampleflow/connections.h>
 
 #include <sstream>
+#include <iostream>
+#include <functional>
+#include <ranges>
+
 
 int main ()
 {
diff --git a/tests/connections_03.cc b/tests/connections_03.cc
index 418d970..4b806fc 100644
--- a/tests/connections_03.cc
+++ b/tests/connections_03.cc
@@ -29,15 +29,15 @@
 // disappearing objects.
 
 
-#include <iostream>
-#include <functional>
-
 #include <sampleflow/producers/range.h>
 #include <sampleflow/filters/condition.h>
 #include <sampleflow/consumers/stream_output.h>
 #include <sampleflow/connections.h>
 
 #include <sstream>
+#include <iostream>
+#include <functional>
+#include <ranges>
 
 int main ()
 {
diff --git a/tests/range_producer_03.cc b/tests/range_producer_03.cc
index 395993e..279aa91 100644
--- a/tests/range_producer_03.cc
+++ b/tests/range_producer_03.cc
@@ -19,11 +19,11 @@
 // class.
 
 
-#include <iostream>
-
 #include <sampleflow/producers/range.h>
 #include <sampleflow/consumers/stream_output.h>
 
+#include <iostream>
+#include <ranges>
 
 
 int main ()
diff --git a/tests/range_producer_04.cc b/tests/range_producer_04.cc
index f709b16..30c8458 100644
--- a/tests/range_producer_04.cc
+++ b/tests/range_producer_04.cc
@@ -19,11 +19,11 @@
 // class.
 
 
-#include <iostream>
-
 #include <sampleflow/producers/range.h>
 #include <sampleflow/consumers/stream_output.h>
 
+#include <iostream>
+#include <ranges>
 
 
 int main ()