Internally, IFFParse maintains iff nesting and scoping context via a
context stack.  The pushchunk() and popchunk() functions get their names
from this basic idea of the iffparse.library.  Direct access to this stack
is not allowed.  However, many functions are provided to assist in
examining and manipulating the context stack.

    About the Context Stack.
    ------------------------
    It is probably easier to think of a stack of blocks on a table in
    front of you when reading this discussion.

As the nesting level increases (as would happen when parsing a nested list
or form), the depth of the context stack increases; new elements are added
to the top.  When these contexts expire, the contextnodes are deleted and
the stack shrinks.

 context nodes 
 the default context 
 context-specific data: localcontextitems 
 storing lcis 
 some interesting internal details