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.

operand-list.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/array"
 )
 
@@ -17,7 +18,7 @@ func newListTermA(args ...*scan.Term) *scan.Term {
 
 func newListTerm(row, col int, args []*scan.Term) *scan.Term {
 	return &scan.Term{
-		Tk:       *scan.NewValueToken(row, col, scan.SymList, "[]", args),
+		Tk:       *scan.NewValueToken(row, col, sym.SymList, "[]", args),
 		Parent:   nil,
 		Children: nil,
 		Position: scan.PosLeaf,