ds={
		"init":func(end){@end=end; @current=0 but true},
		"current":func(){current},
		"next":func(){
			((next=current+1) <= end) ? [true] {@current=next but current} :: {nil}
		}
		,		"filter":func(item,index) { item > 0 }
		,		"map":   func(item,index) { item * 2 }
}

/* Example
;
it=$(ds,3);
add(it)
*/