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.

types/interval/interval.go

@@ -0,0 +1,68 @@
+// Copyright (c) 2024-2026 Celestino Amoroso (celestino.amoroso@gmail.com).
+// All rights reserved.
+
+// interval.go
+package interval
+
+import (
+	"fmt"
+
+	"git.portale-stac.it/go-pkg/expr/kern"
+)
+
+const TypeName = "interval"
+
+type IntervalType struct {
+	begin, end, step int64
+}
+
+func NewInterval(begin, end, step int64) *IntervalType {
+	return &IntervalType{
+		begin: begin,
+		end:   end,
+		step:  step,
+	}
+}
+
+func (p *IntervalType) Begin() int64 {
+	return p.begin
+}
+
+func (p *IntervalType) End() int64 {
+	return p.end
+}
+
+func (p *IntervalType) Step() int64 {
+	return p.step
+}
+
+func IsInterval(v any) (ok bool) {
+	_, ok = v.(*IntervalType)
+	return ok
+}
+
+func (p *IntervalType) TypeName() string {
+	return TypeName
+}
+
+func (p *IntervalType) ToString(opt kern.FmtOpt) string {
+	return fmt.Sprintf("%d..%d..%d", p.begin, p.end, p.step)
+}
+
+func (p *IntervalType) String() string {
+	return p.ToString(0)
+}
+
+func (p *IntervalType) ToIntTriple() (b, e, s int) {
+	b = int(p.begin)
+	e = int(p.end)
+	s = int(p.step)
+	return
+}
+
+// func (b *bool) EqualTo(other kern.Equaler) (equal bool) {
+// 	if otherBool, ok := other.(*bool); ok {
+// 		equal = b == otherBool
+// 	}
+// 	return
+// }