return the :dimensions from the header of a 1- or 2-dimensional array
Major Section:  ARRAYS

Example Form:
(dimensions 'delta1 a)

General Form: (dimensions name alist)

where name is arbitrary and alist is a 1- or 2-dimensional array. This function returns the dimensions list of the array alist. That list will either be of the form (dim1) or (dim1 dim2), depending on whether alist is a 1- or 2-dimensional array. Dim1 and dim2 will be integers and each exceed by 1 the maximum legal corresponding index. Thus, if dimensions returns, say, '(100) for an array a named 'delta1, then (aref1 'delta1 a 99) is legal but (aref1 'delta1 a 100) violates the guards on aref1. Dimensions operates in virtually constant time if alist is the semantic value of name. See arrays.