several changes:

1. moved scan/symbol.go to new package sym and adapted all files that reference Symbol type and its values.
2. new type interval defined by begin, end and step values.
3. replaced operator-range.go with operator-interval.go.
4. replaced range operator begin:end with begin..end..step.
5. new implementation of sub-collection extraction based on new interval literal.

t_list_test.go

@@ -19,7 +19,7 @@ func TestLinkedListParser(t *testing.T) {
 		/*   2 */ {`it=$([<1,2,3>]); it++`, int64(1), nil},
 		/*   3 */ {`it=$([<1,2,3>]); #($$(it))`, int64(3), nil},
 		/*   4 */ {`[<1,2,3>][0]`, int64(1), nil},
-		/*   5 */ {`[<1,2,3>][0:2]`, list.NewLinkedListA(1, 2), nil},
+		/*   5 */ {`[<1,2,3>][0..2]`, list.NewLinkedListA(1, 2), nil},
 		/*   6 */ {`[<1>] + [<2,3>]`, list.NewLinkedListA(1, 2, 3), nil},
 		/*   7 */ {`L=[<1>] + [<2,3>]; L[0]=100; L`, list.NewLinkedListA(100, 2, 3), nil},
 		/*   8 */ {`builtin "base"; L=[<1>]; isList(L)`, true, nil},