< list > < n > )
Return the elements of `list' grouped in sublists of length `n,' which should
be a positive integer. For example, the list '(a b c d) grouped by 2 yields
'((a b) (c d)). Definition of `group' taken from On Lisp, by Paul Graham. The
definition is not hugely efficient, so this function is appropriate for parsing
macro args, but no for inner loops.