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-shift.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"
 )
 
@@ -82,6 +83,6 @@ func evalLeftShift(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error) {
 
 // init
 func init() {
-	scan.RegisterTermConstructor(scan.SymDoubleGreater, newRightShiftTerm)
-	scan.RegisterTermConstructor(scan.SymDoubleLess, newLeftShiftTerm)
+	scan.RegisterTermConstructor(sym.SymDoubleGreater, newRightShiftTerm)
+	scan.RegisterTermConstructor(sym.SymDoubleLess, newLeftShiftTerm)
 }