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.

scan/term.go

@@ -8,13 +8,14 @@ import (
 	"strings"
 
 	"git.portale-stac.it/go-pkg/expr/kern"
+	"git.portale-stac.it/go-pkg/expr/sym"
 )
 
 type TermPriority uint32
 
 const (
 	PriNone TermPriority = iota
-	PriRange
+	PriInterval
 	PriBut
 	PriAssign
 	PriIterOp // map, filter, digest, etc
@@ -160,7 +161,7 @@ func (t *Term) SetParent(parent *Term) {
 	}
 }
 
-func (t *Term) Symbol() Symbol {
+func (t *Term) Symbol() sym.Symbol {
 	return t.Tk.Sym
 }
 
@@ -200,12 +201,24 @@ func (t *Term) GetRightChild() (c kern.Term) {
 	return
 }
 
+func (t *Term) IsSymbol(symbol sym.Symbol) bool {
+	return t.Symbol() == symbol
+}
+
+func (t *Term) IsOneOf(symbols ...sym.Symbol) bool {
+	return t.Tk.IsOneOf(symbols)
+}
+
 func (t *Term) IsAssign() bool {
-	return t.Symbol() == SymEqual
+	return t.Symbol() == sym.SymEqual
 }
 
 func (t *Term) IsVar() bool {
-	return t.Symbol() == SymVariable
+	return t.Symbol() == sym.SymVariable
+}
+
+func (t *Term) SetSymbol(symbol sym.Symbol) {
+	t.Tk.SetSymbol(symbol)
 }
 
 // func (t *Term) IsFunc() bool {