diff --git a/histogram.cpp b/histogram.cpp
index 4d33c05..77c4a03 100644
--- a/histogram.cpp
+++ b/histogram.cpp
@@ -2,7 +2,7 @@
 
 using namespace std;
 
-static void
+void
 find_minmax(const vector<double>& numbers, double& min, double& max)
 {
     min = numbers[0];
diff --git a/histogram_internal.h b/histogram_internal.h
new file mode 100644
index 0000000..34592d8
--- /dev/null
+++ b/histogram_internal.h
@@ -0,0 +1,9 @@
+#ifndef HISTOGRAM_INTERNAL_H_INCLUDED
+#define HISTOGRAM_INTERNAL_H_INCLUDED
+
+#include <vector>
+
+void
+find_minmax(const std::vector<double>& numbers, double& min, double& max)
+
+#endif // HISTOGRAM_INTERNAL_H_INCLUDED
diff --git a/text.h b/text.h
index 5733dff..5c6df4d 100644
--- a/text.h
+++ b/text.h
@@ -1,6 +1,8 @@
 #ifndef TEXT_H_INCLUDED
 #define TEXT_H_INCLUDED
 
+#include <vector>
+
 void
 show_histogram_text(const std::vector<size_t>& bins, size_t& bin_count);