samer@10: % bracket_it - Bracket iterator with pre and post actions
samer@10: %
samer@10: % bracket_it :: 
samer@10: %    iterator(A) ~'an iterator', 
samer@10: %    (A=>void)   ~'action to perform before calling iterator',
samer@10: %    (A=>void)   ~'action to perform after calling iterator'
samer@10: % -> iterator(A) ~'new iterator incorporating pre and post actions'.
samer@10: function it=bracket_it(it,pre,post)
samer@10: 	f=it{1}; it{1}=@bf;
samer@10: 	function x=bf(x), pre(x); x=f(x); post(x); end
samer@10: end