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_token_test.go

@@ -9,19 +9,20 @@ import (
 	"testing"
 
 	"git.portale-stac.it/go-pkg/expr/scan"
+	"git.portale-stac.it/go-pkg/expr/sym"
 )
 
 func TestDevString(t *testing.T) {
 	type inputType struct {
 		source     string
-		sym        scan.Symbol
+		sym        sym.Symbol
 		value      any
 		wantResult string
 	}
 
 	inputs := []inputType{
-		/*   1 */ {"100", scan.SymInteger, 100, fmt.Sprintf(`[%d]"100"{100}`, scan.SymInteger)},
-		/*   2 */ {"+", scan.SymPlus, nil, fmt.Sprintf(`[%d]"+"{}`, scan.SymPlus)},
+		/*   1 */ {"100", sym.SymInteger, 100, fmt.Sprintf(`[%d]"100"{100}`, sym.SymInteger)},
+		/*   2 */ {"+", sym.SymPlus, nil, fmt.Sprintf(`[%d]"+"{}`, sym.SymPlus)},
 	}
 
 	for i, input := range inputs {