remove irrelevant pairs from a 2-dimensional array
Major Section:  ARRAYS

Example Form:
(compress2 'delta1 a)

General Form:
(compress2 name alist)
where name is a symbol and alist is a 2-dimensional array, generally named name. See arrays for details. Logically speaking, this function removes irrelevant pairs from alist, possibly shortening it. The function returns a new array, alist', with the same header (including name and dimension) as alist, that, under aref2, is everywhere equal to alist. That is, (aref2 name alist' i j) is (aref2 name alist i j), for all legal indices i and j. Alist' may be shorter than alist and the non-irrelevant pairs may occur in a different order in alist' than in alist.

Practically speaking, this function plays an important role in the efficient implementation of aref2. In addition to creating the new array, alist', compress2 makes that array the ``semantic value'' of name and allocates a raw lisp array to name. For all legal indices, i and j, that raw lisp array contains (aref2 name alist' i j) in slot i,j. Thus, subsequent aref2 operations can be executed in virtually constant time provided they are given name and the alist' returned by the most recently executed compress2 or aset2 on name. See arrays.