Added a note that objects which emulate built-in types should only
implement as many of the relevant methods as make sense for the particular
information being modelled.
diff --git a/Doc/ref/ref3.tex b/Doc/ref/ref3.tex
index 905a001..0222110 100644
--- a/Doc/ref/ref3.tex
+++ b/Doc/ref/ref3.tex
@@ -841,6 +841,13 @@
operation raise an exception when no appropriate method is defined.
\withsubitem{(mapping object method)}{\ttindex{__getitem__()}}
+When implementing a class that emulates any built-in type, it is
+important that the emulation only be implemented to the degree that it
+makes sense for the object being modelled. For example, some
+sequences may work well with retrieval of individual elements, but
+extracting a slice may not make sense. (One example of this is the
+\class{NodeList} interface in the W3C's Document Object Model.)
+
\subsection{Basic customization\label{customization}}