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-expr.go

@@ -9,11 +9,12 @@ 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"
 )
 
 // -------- expr term
 func newExprTerm(root *scan.Term) *scan.Term {
-	tk := scan.NewValueToken(root.Tk.Row(), root.Tk.Col(), scan.SymExpression, root.Source(), root)
+	tk := scan.NewValueToken(root.Tk.Row(), root.Tk.Col(), sym.SymExpression, root.Source(), root)
 	return &scan.Term{
 		Tk:       *tk,
 		Parent:   nil,