slice can accept negative indices

docs/scarpet/language/Containers.md

@@ -170,14 +170,21 @@ split('\\.','foo.bar')  => [foo, bar]
 ### `slice(expr, from, to?)`
 
 extracts a substring, or sublist (based on the type of the result of the expression under expr with 
-starting index of `from`, and ending at `to` if provided, or the end, if omitted
+starting index of `from`, and ending at `to` if provided, or the end, if omitted. Can use negative indices to 
+indicate counting form the back of the list, so `-1 <=> length(_)`.
+
+Special case is made for iterators (`range`, `rect` etc), which does require non-negative indices (negative `from` is treated as
+`0`, and negative `to` as `inf`), but allows retrieving parts of the sequence and ignore
+other parts. In that case consecutive calls to `slice` will refer to index `0` the current iteration position since iterators
+cannot go back nor track where they are in the sequence (see examples).
 
 <pre>
-slice(l(0,1,2,3,4,5), 1, 3)  => [1, 2, 3]
+slice([0,1,2,3,4,5], 1, 3)  => [1, 2, 3]
 slice('foobar', 0, 1)  => 'f'
 slice('foobar', 3)  => 'bar'
 slice(range(10), 3, 5)  => [3, 4, 5]
 slice(range(10), 5)  => [5, 6, 7, 8, 9]
+r = range(100); [slice(r, 5, 7), slice(r, 1, 3)]  => [[5, 6], [8, 9]]
 </pre>
 
 ### `sort(list), sort(values ...)`