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.

builtin-import.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"
 )
 
 func importFunc(ctx kern.ExprContext, name string, args map[string]any) (result any, err error) {
@@ -47,7 +48,7 @@ func doImport(ctx kern.ExprContext, name string, dirList []string, it kern.Itera
 			var expr *scan.Ast
 			scanner := scan.NewScanner(file, scan.DefaultTranslations())
 			parser := NewParser()
-			if expr, err = parser.parseGeneral(scanner, allowMultiExpr|allowVarRef, scan.SymEos); err == nil {
+			if expr, err = parser.parseGeneral(scanner, allowMultiExpr|allowVarRef, sym.SymEos); err == nil {
 				result, err = expr.Eval(ctx)
 			}
 			if err != nil {