number of stack narrowing specifier pairs. The first specifier in
a pair controls inner border, the second the outer border. A
number means cut that number of frames, a procedure object means
cut until that object is found in operator position in a frame.
a stack object. Introduce one level of indirection in the stack
object to make it possible to "narrow" to a certain region of the
stack. This facilitates making use of more clever algorithms (not
implemented) for selecting relevant frames and gives a cleaner
design since selection of frames can be done independently of
extraction of frames from the real stack.
(scm_stack_id): Also take #t as argument which means look at
current stack.
