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-fraction.go

@@ -11,6 +11,7 @@ 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/fract"
 )
 
@@ -72,5 +73,5 @@ func evalFraction(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error) {
 // init
 func init() {
 	// registerTermConstructor(SymVertBar, newFractionTerm)
-	scan.RegisterTermConstructor(scan.SymColon, newFractionTerm)
+	scan.RegisterTermConstructor(sym.SymColon, newFractionTerm)
 }