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-iterator.go

@@ -10,6 +10,7 @@ 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/list"
@@ -18,7 +19,7 @@ import (
 // -------- iterator term
 
 func newIteratorTerm(tk *scan.Token, args []*scan.Term) *scan.Term {
-	tk.Sym = scan.SymIterator
+	tk.Sym = sym.SymIterator
 	return &scan.Term{
 		Tk:       *tk,
 		Parent:   nil,