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-plugin.go

@@ -7,6 +7,7 @@ package expr
 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"
 )
 
 //-------- plugin term
@@ -37,5 +38,5 @@ func evalPlugin(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error) {
 
 // init
 func init() {
-	scan.RegisterTermConstructor(scan.SymKwPlugin, newPluginTerm)
+	scan.RegisterTermConstructor(sym.SymKwPlugin, newPluginTerm)
 }