A stack is a cursor onto a window list. The data structure tracks
focus by construction, and the master window is by convention the
top-most item. Focus operations will not reorder the list that results
from flattening the cursor. The structure can be envisaged as:
+-- master: < '7' >
up | [ '2' ]
+--------- [ '3' ]
focus: < '4' >
dn +----------- [ '8' ]
A
Stack can be viewed as a list with a hole punched in it to
make the focused position. Under the zipper/calculus view of such
structures, it is the differentiation of a [a], and integrating it
back has a natural implementation used in
index.