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-var.go

@@ -9,6 +9,7 @@ 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"
 )
 
 // -------- variable term
@@ -21,7 +22,7 @@ func newVarTerm(tk *scan.Token) *scan.Term {
 		Priority: scan.PriValue,
 		EvalFunc: evalVar,
 	}
-	t.Tk.Sym = scan.SymVariable
+	t.Tk.Sym = sym.SymVariable
 	return t
 }
 
@@ -41,5 +42,5 @@ func evalVar(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error) {
 
 // init
 func init() {
-	scan.RegisterTermConstructor(scan.SymIdentifier, newVarTerm)
+	scan.RegisterTermConstructor(sym.SymIdentifier, newVarTerm)
 }