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-groupby.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/array"
 	"git.portale-stac.it/go-pkg/expr/types/dict"
 	"git.portale-stac.it/go-pkg/expr/types/str"
@@ -51,7 +52,7 @@ func evalGroupBy(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error) {
 	}
 
 	rightTk := opTerm.Children[1].Tk
-	if rightTk.IsSymbol(scan.SymVariable) && rightTk.Source() == "__" {
+	if rightTk.IsSymbol(sym.SymVariable) && rightTk.Source() == "__" {
 		keyByIndex = true
 	} else if rightValue, err = opTerm.Children[1].Compute(ctx); err != nil {
 		return
@@ -112,5 +113,5 @@ func evalGroupBy(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error) {
 
 // init
 func init() {
-	scan.RegisterTermConstructor(scan.SymKwGroupBy, newGroupByTerm)
+	scan.RegisterTermConstructor(sym.SymKwGroupBy, newGroupByTerm)
 }