Like custom stream handlers, custom chunk handlers are implemented using
Release 2 hook structures.  see the previous section for details on how a
handler function may be interfaced using a Hook structure.

There are two types of chunk handlers:  entry handlers and exit handlers.
Entry handlers are invoked just after the parser enters the chunk; the
stream will be positioned to read the first byte in the chunk. (If the
chunk is a form, list, cat, or prop, the longword type will be read by the
parser; the stream will be positioned to read the byte immediately
following the type.)  Exit handlers are invoked just before the parser
leaves the chunk; the stream is not positioned predictably within the
chunk.

 installing a custom chunk handler 
 inside a custom chunk handler 
 the object parameter