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-interval.go

@@ -0,0 +1,103 @@
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
+// All rights reserved.
+
+// operator-interval.go
+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"
+	"git.portale-stac.it/go-pkg/expr/types/interval"
+)
+
+// -------- interval term
+
+func newIntervalTerm(tk *scan.Token) (inst *scan.Term) {
+	return &scan.Term{
+		Tk:       *tk,
+		Children: make([]*scan.Term, 0, 2),
+		Position: scan.PosInfix,
+		Priority: scan.PriInterval,
+		EvalFunc: evalInterval,
+	}
+}
+
+func changeColonToInterval(t *scan.Term) {
+	if t.IsSymbol(sym.SymDoubleDot) {
+		t.SetSymbol(sym.SymInterval)
+		t.EvalFunc = evalInterval
+		if child := t.GetLeftChild(); child != nil && child.IsSymbol(sym.SymDoubleDot) {
+			child.SetSymbol(sym.SymInterval)
+		}
+	}
+}
+
+func evalInterval(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error) {
+	var begin, end int64
+
+	step := int64(1)
+	switch opTerm.GetChildCount() {
+	case 0:
+		begin = 0
+		end = int64(kern.ConstLastIndex)
+	case 1:
+		var leftValue any
+		if leftValue, err = opTerm.GetChild(0).Compute(ctx); err != nil {
+			return
+		}
+		if begin, err = checkIntervalComponent(opTerm, leftValue); err != nil {
+			return
+		}
+		end = int64(kern.ConstLastIndex)
+	default:
+		var beginValue, endValue, stepValue any
+		subTerm := opTerm.GetLeftChild()
+		if subTerm.IsOneOf(sym.SymInterval, sym.SymDoubleDot) {
+			if beginValue, endValue, err = subTerm.EvalInfix(ctx); err != nil {
+				return
+			}
+			if stepValue, err = opTerm.GetRightChild().Compute(ctx); err != nil {
+				return
+			}
+			step, err = checkIntervalComponent(opTerm.GetRightChild(), stepValue)
+		} else if beginValue, endValue, err = opTerm.EvalInfix(ctx); err != nil {
+			return
+		}
+		if begin, err = checkIntervalComponent(opTerm, beginValue); err != nil {
+			return
+		}
+		if end, err = checkIntervalComponent(opTerm, endValue); err != nil {
+			return
+		}
+	}
+
+	if step <= 0 {
+		err = opTerm.Errorf("invalid %s specification: step must be positive", interval.TypeName)
+	} else {
+		v = interval.NewInterval(begin, end, step)
+	}
+	return
+}
+
+func checkIntervalComponent(t kern.Term, value any) (component int64, err error) {
+	if component, err = types.ToGoInt64(value, "%s expression"); err != nil {
+		err = t.Errorf(err.Error(), interval.TypeName)
+	}
+	return
+}
+
+func errIntervalInvalidSpecification(t kern.Term) error {
+	return t.Errorf("invalid %s specification", interval.TypeName)
+}
+
+func errIntervalUnexpectedExpression(t *scan.Term) error {
+	return t.Errorf("unexpected %s expression", interval.TypeName)
+}
+
+// init
+func init() {
+	scan.RegisterTermConstructor(sym.SymInterval, newIntervalTerm)
+	scan.RegisterTermConstructor(sym.SymDoubleDot, newIntervalTerm)
+}