return the :maximum-length from the header of an array
Major Section:  ARRAYS

Example Form:
(maximum-length 'delta1 a)

General Form: (maximum-length name alist)

where name is an arbitrary object and alist is a 1- or 2-dimensional array. This function returns the contents of the :maximum-length field of the header of alist. Whenever an aset1 or aset2 would cause the length of the alist to exceed its maximum length, a compress1 or compress2 is done automatically to remove irrelevant pairs from the array. Maximum-length operates in virtually constant time if alist is the semantic value of name. See arrays.