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-constructor-registry.go

@@ -4,17 +4,19 @@
 // term-constructor-registry.go
 package scan
 
+import "git.portale-stac.it/go-pkg/expr/sym"
+
 const initialRegistryCapacity = 10
 
 type termContructor func(tk *Token) *Term
 
-var constructorRegistry map[Symbol]termContructor = nil
+var constructorRegistry map[sym.Symbol]termContructor = nil
 
-func RegisterTermConstructor(sym Symbol, constructor termContructor) {
+func RegisterTermConstructor(symmbol sym.Symbol, constructor termContructor) {
 	if constructorRegistry == nil {
-		constructorRegistry = make(map[Symbol]termContructor, initialRegistryCapacity)
+		constructorRegistry = make(map[sym.Symbol]termContructor, initialRegistryCapacity)
 	}
-	constructorRegistry[sym] = constructor
+	constructorRegistry[symmbol] = constructor
 }
 
 func NewTerm(tk *Token) (inst *Term) {