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_dict_test.go

@@ -11,6 +11,7 @@ import (
 
 	"git.portale-stac.it/go-pkg/expr/kern"
 	"git.portale-stac.it/go-pkg/expr/scan"
+	"git.portale-stac.it/go-pkg/expr/sym"
 	"git.portale-stac.it/go-pkg/expr/types/dict"
 	"git.portale-stac.it/go-pkg/expr/types/list"
 )
@@ -109,7 +110,7 @@ func TestDictToStringMultiLine(t *testing.T) {
   "first": 1
 }`
 	args := map[any]any{
-		"first": newLiteralTerm(scan.NewValueToken(0, 0, scan.SymInteger, "1", 1)),
+		"first": newLiteralTerm(scan.NewValueToken(0, 0, sym.SymInteger, "1", 1)),
 	}
 	dict := dict.NewDict(args)
 	got := dict.ToString(kern.MultiLine)
@@ -131,7 +132,7 @@ func TestDictToString(t *testing.T) {
 	section := "dict-ToString-SL"
 	want := `{"first": 1}`
 	args := map[any]any{
-		"first": newLiteralTerm(scan.NewValueToken(0, 0, scan.SymInteger, "1", 1)),
+		"first": newLiteralTerm(scan.NewValueToken(0, 0, sym.SymInteger, "1", 1)),
 	}
 	dict := dict.NewDict(args)
 	got := dict.ToString(0)