doc

doc/extending/cop.txt

@@ -33,6 +33,11 @@ There are less methods to define for an Op than for a Type:
 
       This must return C code that carries the computation we want to do.
 
+      sub is a dictionary of strings for you to substitute into your code.
+      It's not clear if it ever contains anything other than 'fail'.
+      sub['fail'] is a string of code that you should execute (after calling
+      PyErr_Format) if your C code needs to raise an exception.
+
     .. method:: c_code_cleanup(node, name, input_names, output_names, sub)
 
       This must return C code that cleans up whatever c_code allocated and