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-default.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"
 )
 
 //-------- default term
@@ -29,7 +30,7 @@ func evalDefault(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error) {
 	}
 
 	leftTerm := opTerm.Children[0]
-	if leftTerm.Tk.Sym != scan.SymVariable {
+	if leftTerm.Tk.Sym != sym.SymVariable {
 		// err = leftTerm.Errorf("left operand of %q must be a variable", self.tk.source)
 		err = kern.ErrLeftOperandMustBeVariable(leftTerm, opTerm)
 		return
@@ -63,7 +64,7 @@ func evalAlternate(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error) {
 	}
 
 	leftTerm := opTerm.Children[0]
-	if leftTerm.Tk.Sym != scan.SymVariable {
+	if leftTerm.Tk.Sym != sym.SymVariable {
 		// err = leftTerm.Errorf("left operand of %q must be a variable", self.tk.source)
 		err = kern.ErrLeftOperandMustBeVariable(leftTerm, opTerm)
 		return
@@ -99,7 +100,7 @@ func evalAssignDefault(ctx kern.ExprContext, opTerm *scan.Term) (v any, err erro
 	}
 
 	leftTerm := opTerm.Children[0]
-	if leftTerm.Tk.Sym != scan.SymVariable {
+	if leftTerm.Tk.Sym != sym.SymVariable {
 		// err = leftTerm.Errorf("left operand of %q must be a variable", self.tk.source)
 		err = kern.ErrLeftOperandMustBeVariable(leftTerm, opTerm)
 		return
@@ -123,7 +124,7 @@ func evalAssignDefault(ctx kern.ExprContext, opTerm *scan.Term) (v any, err erro
 
 // init
 func init() {
-	scan.RegisterTermConstructor(scan.SymDoubleQuestion, newDefaultTerm)
-	scan.RegisterTermConstructor(scan.SymQuestionEqual, newDefaultAssignTerm)
-	scan.RegisterTermConstructor(scan.SymQuestionExclam, newAlternateTerm)
+	scan.RegisterTermConstructor(sym.SymDoubleQuestion, newDefaultTerm)
+	scan.RegisterTermConstructor(sym.SymQuestionEqual, newDefaultAssignTerm)
+	scan.RegisterTermConstructor(sym.SymQuestionExclam, newAlternateTerm)
 }