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+*********************
+Type System Variables
+*********************
+
+User written code can be placed in arbitrary places using the
+:doc:`inject-code <codeinjectionsemantics>` tag. To ease the binding developer
+work, the injected code can make use of special variables that will be replaced
+by the correct values. This also shields the developer from some |project|
+implementation specifics.
+
+
+Variables
+=========
+
+**%0**
+
+  Replaced by the name of the return variable of the Python method/function wrapper.
+
+
+**%#**
+
+  Replaced by the name of a C++ argument in the position indicated by ``#``.
+  The argument counting starts with ``%1``, since ``%0`` represents the return
+  variable name.
+
+
+**%ARGUMENT_NAMES**
+
+  Replaced by a comma separated list with the names of all arguments that were
+  not removed on the type system description for the method/function.
+
+
+**%CONVERTTOPYTHON[CPPTYPE]**
+
+  Replaced by a |project| conversion call that converts a C++ variable of the
+  type indicated by ``CPPTYPE`` to the proper Python object.
+
+
+**%CPPSELF**
+
+  Replaced by the wrapped C++ object instance that owns the method in which the
+  code with this variable was inserted.
+
+
+**%FUNCTION_NAME**
+
+  Replaced by the name of a function or method.
+
+
+**%PYARG_#**
+
+  Similar to ``%#``, but is replaced by the Python arguments (PyObjects)
+  received by the Python wrapper method.
+
+
+**%PYSELF**
+
+  Replaced by the Python wrapper variable (a PyObject) representing the instance
+  bounded to the Python wrapper method which receives the custom code.
+
+
+**%RETURN_TYPE**
+
+  Replaced by the type returned by a function or method.
+
+
+**%TYPE**
+
+  Replaced by the name of the class to which a function belongs. Should be used
+  in code injected to methods.
+
+
+Example
+=======
+
+Just to illustrate the usage of the variables described in the previous
+sections, below is an excerpt from the type system description of a |project|
+test. It changes a method that received ``argc/argv`` arguments into something
+that expects a Python sequence instead.
+
+    .. code-block:: xml
+
+        <modify-function signature="overloadedMethod(int, char**)">
+            <modify-argument index="1">
+                <replace-type modified-type="PySequence" />
+            </modify-argument>
+            <modify-argument index="2">
+                <remove-argument />
+            </modify-argument>
+            <inject-code class="target" position="beginning">
+                int argc;
+                char** argv;
+                if (!PySequence_to_argc_argv(%PYARG_1, &amp;argc, &amp;argv)) {
+                    PyErr_SetString(PyExc_TypeError, "error");
+                    return 0;
+                }
+                %RETURN_TYPE foo = %CPPSELF.%FUNCTION_NAME(argc, argv);
+                %0 = %CONVERTTOPYTHON[%RETURN_TYPE](foo);
+
+                for (int i = 0; i &lt; argc; ++i)
+                    delete[] argv[i];
+                delete[] argv;
+            </inject-code>
+        </modify-function>
+