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-filter.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/boolean"
 )
 
@@ -164,5 +165,5 @@ func (it *filterIterator) CallOperation(name string, args map[string]any) (v any
 
 // init
 func init() {
-	scan.RegisterTermConstructor(scan.SymKwFilter, newFilterTerm)
+	scan.RegisterTermConstructor(sym.SymKwFilter, newFilterTerm)
 }