diff --git a/examples/extern_C/README b/examples/extern_C/README
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+++ b/examples/extern_C/README
@@ -0,0 +1,6 @@
+This example show how to import an external function written in C in an Heptagon program.
+
+- The .epi file contains the signature of the function in Heptagon.
+- Only functions with no side effects can be safely imported.
+- The imported function must have the same calling convention as generated code (inputs given by value, outputs in a f_out structure).
+- The generated code will include "mathext.h", that should define the step function
diff --git a/examples/extern_C/imports.ept b/examples/extern_C/imports.ept
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+++ b/examples/extern_C/imports.ept
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+open Mathext
+
+fun f(a:float) returns (o:float)
+let
+  o = mycos(a);
+tel
\ No newline at end of file
diff --git a/examples/extern_C/mathext.c b/examples/extern_C/mathext.c
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+++ b/examples/extern_C/mathext.c
@@ -0,0 +1,7 @@
+#include <math.h>
+#include "mathext.h"
+
+void mycos(float a, mycos_out *out)
+{
+  out->o = cos(a);
+}
diff --git a/examples/extern_C/mathext.epi b/examples/extern_C/mathext.epi
new file mode 100644
index 0000000..3eb4f32
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+++ b/examples/extern_C/mathext.epi
@@ -0,0 +1 @@
+val fun mycos(a:float) returns (o:float)
\ No newline at end of file
diff --git a/examples/extern_C/mathext.h b/examples/extern_C/mathext.h
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+++ b/examples/extern_C/mathext.h
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+#ifdef MATHEXT_H
+#define MATHEXT_H
+
+struct mycos_out {
+  float o;
+};
+
+void mycos(float a, mycos_out *o);