system.shallow documented

doc/nimrodc.txt

@@ -419,6 +419,18 @@ For ``let`` symbols a copy is not always necessary:
   let s = varA    # may only copy a pointer if it safe to do so
 
 
+If you know what you're doing, you can also mark single string (or sequence)
+objects as `shallow`:idx:\:
+
+.. code-block:: Nimrod
+  var s = "abc"
+  shallow(s) # mark 's' as shallow string
+  var x = s  # now might does not copy the string!
+  
+Usage of ``shallow`` is always safe once you know the string won't be modified
+anymore, similar to Ruby's `freeze`:idx:.
+
+
 The compiler optimizes string case statements: A hashing scheme is used for them
 if several different string constants are used. So code like this is reasonably
 efficient: