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-context.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"
 	"git.portale-stac.it/go-pkg/expr/types/list"
 )
 
@@ -30,7 +31,7 @@ func evalContextValue(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error
 
 	if len(opTerm.Children) == 0 {
 		sourceCtx = ctx
-	} else if opTerm.Children[0].Symbol() == scan.SymVariable && opTerm.Children[0].Source() == "global" {
+	} else if opTerm.Children[0].Symbol() == sym.SymVariable && opTerm.Children[0].Source() == "global" {
 		sourceCtx = ctx.GetGlobal()
 	} else if childValue, err = opTerm.EvalPrefix(ctx); err == nil {
 		if dc, ok := childValue.(*dataCursor); ok {
@@ -66,5 +67,5 @@ func evalContextValue(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error
 
 // init
 func init() {
-	scan.RegisterTermConstructor(scan.SymDoubleDollar, newContextTerm)
+	scan.RegisterTermConstructor(sym.SymDoubleDollar, newContextTerm)
 }