Add an example of builtin function embedding into a user function

doc/userdoc.org

@@ -2295,6 +2295,18 @@ functions. They differ from user-defined functions in three points.
     (actually, this is not a limitation since it is trivial to embed a
     builtin function into a user-defined one).
 
+Here is a simple example of builtin function embedding in a user
+function.
+#+NAME: builtin-function-embedding
+#+BEGIN_SRC pugs :exports both :results output
+  import math;
+
+  let cosinus: R -> R, x -> cos(x);
+  cout << "cosinus(2) = " << cosinus(2) << "\n";
+#+END_SRC
+Running this example produces
+#+results: builtin-function-embedding
+
 Builtin functions are defined when modules are imported, see [[modules]].
 
 ** TODO modules<<modules>>