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.

operator-dot.go

@@ -7,6 +7,7 @@ package expr
 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/util"
 )
@@ -36,7 +37,7 @@ func evalDot(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error) {
 
 	switch unboxedValue := leftValue.(type) {
 	case kern.ExtIterator:
-		if indexTerm.Symbol() == scan.SymVariable /*|| indexTerm.Tk.Sym == scan.SymString */ {
+		if indexTerm.Symbol() == sym.SymVariable /*|| indexTerm.Tk.Sym == sym.SymString */ {
 			opName := indexTerm.Source()
 			if unboxedValue.HasOperation(opName) {
 				v, err = unboxedValue.CallOperation(opName, map[string]any{})
@@ -61,7 +62,7 @@ func dotGetDictItemValue(ctx kern.ExprContext, d *dict.DictType, rightTerm *scan
 	var ok bool
 	var rightValue any
 	s := rightTerm.Symbol()
-	if s == scan.SymVariable || s == scan.SymString {
+	if s == sym.SymVariable || s == sym.SymString {
 		// src := rightTerm.Source()
 		// if len(src) > 1 && src[0] == '"' && src[len(src)-1] == '"' {
 		// 	src = src[1 : len(src)-1]
@@ -87,5 +88,5 @@ func errDictKeyNotFound(term *scan.Term, key any) error {
 
 // init
 func init() {
-	scan.RegisterTermConstructor(scan.SymDot, newDotTerm)
+	scan.RegisterTermConstructor(sym.SymDot, newDotTerm)
 }