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.

kern/term.go

@@ -6,6 +6,8 @@ package kern
 
 import (
 	"fmt"
+
+	"git.portale-stac.it/go-pkg/expr/sym"
 )
 
 type Term interface {
@@ -17,8 +19,12 @@ type Term interface {
 	GetChildSource(index int) string
 	GetLeftChild() (c Term)
 	GetRightChild() (c Term)
+	Symbol() (symbol sym.Symbol)
+	IsSymbol(symbol sym.Symbol) bool
+	IsOneOf(symbols ...sym.Symbol) bool
 	IsAssign() bool
 	IsVar() bool
+	SetSymbol(symbol sym.Symbol)
 	Compute(ctx ExprContext) (result any, err error)
 	EvalInfix(ctx ExprContext) (leftValue, rightValue any, err error)
 	Errorf(template string, args ...any) (err error)