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.

parser.go

@@ -10,6 +10,7 @@ import (
 	"slices"
 
 	"git.portale-stac.it/go-pkg/expr/scan"
+	"git.portale-stac.it/go-pkg/expr/sym"
 	"golang.org/x/exp/constraints"
 )
 
@@ -58,7 +59,7 @@ func NewParser() (p *parser) {
 }
 
 func (parser *parser) Next(scanner *scan.Scanner) (tk *scan.Token) {
-	for tk = scanner.Next(); tk.IsSymbol(scan.SymComment); tk = scanner.Next() {
+	for tk = scanner.Next(); tk.IsSymbol(sym.SymComment); tk = scanner.Next() {
 	}
 	return
 }
@@ -66,10 +67,10 @@ func (parser *parser) Next(scanner *scan.Scanner) (tk *scan.Token) {
 func (parser *parser) parseFuncCall(scanner *scan.Scanner, ctx parserContext, tk *scan.Token) (tree *scan.Term, err error) {
 	args := make([]*scan.Term, 0, 10)
 	itemExpected := false
-	lastSym := scan.SymUnknown
-	for lastSym != scan.SymClosedRound && lastSym != scan.SymEos {
+	lastSym := sym.SymUnknown
+	for lastSym != sym.SymClosedRound && lastSym != sym.SymEos {
 		var subTree *scan.Ast
-		if subTree, err = parser.parseItem(scanner, ctx, scan.SymComma, scan.SymClosedRound); err != nil {
+		if subTree, err = parser.parseItem(scanner, ctx, sym.SymComma, sym.SymClosedRound); err != nil {
 			break
 		}
 		prev := scanner.Previous()
@@ -80,11 +81,11 @@ func (parser *parser) parseFuncCall(scanner *scan.Scanner, ctx parserContext, tk
 			break
 		}
 
-		itemExpected = prev.Sym == scan.SymComma
+		itemExpected = prev.Sym == sym.SymComma
 		lastSym = scanner.Previous().Sym
 	}
 	if err == nil {
-		if lastSym != scan.SymClosedRound {
+		if lastSym != sym.SymClosedRound {
 			err = errors.New("unterminated arguments list")
 		} else {
 			tree = newFuncCallTerm(tk, args)
@@ -97,13 +98,13 @@ func (parser *parser) parseFuncDef(scanner *scan.Scanner) (tree *scan.Term, err
 	// Example: "add = func(x,y) {x+y}
 	var body *scan.Ast
 	args := make([]*scan.Term, 0)
-	lastSym := scan.SymUnknown
+	lastSym := sym.SymUnknown
 	defaultParamsStarted := false
 	itemExpected := false
 	tk := scanner.Previous()
-	for lastSym != scan.SymClosedRound && lastSym != scan.SymEos {
+	for lastSym != sym.SymClosedRound && lastSym != sym.SymEos {
 		tk = parser.Next(scanner)
-		if tk.IsSymbol(scan.SymIdentifier) {
+		if tk.IsSymbol(sym.SymIdentifier) {
 			param := scan.NewTerm(tk)
 			if len(args) > 0 {
 				if pos := paramAlreadyDefined(args, param); pos > 0 {
@@ -113,10 +114,10 @@ func (parser *parser) parseFuncDef(scanner *scan.Scanner) (tree *scan.Term, err
 			}
 			args = append(args, param)
 			tk = parser.Next(scanner)
-			if tk.Sym == scan.SymEqual {
+			if tk.Sym == sym.SymEqual {
 				var paramExpr *scan.Ast
 				defaultParamsStarted = true
-				if paramExpr, err = parser.parseItem(scanner, parserNoFlags, scan.SymComma, scan.SymClosedRound); err != nil {
+				if paramExpr, err = parser.parseItem(scanner, parserNoFlags, sym.SymComma, sym.SymClosedRound); err != nil {
 					break
 				}
 				param.ForceChild(paramExpr.Root())
@@ -130,25 +131,25 @@ func (parser *parser) parseFuncDef(scanner *scan.Scanner) (tree *scan.Term, err
 			break
 		}
 		lastSym = scanner.Previous().Sym
-		itemExpected = lastSym == scan.SymComma
+		itemExpected = lastSym == sym.SymComma
 	}
 
-	if err == nil && lastSym != scan.SymClosedRound {
+	if err == nil && lastSym != sym.SymClosedRound {
 		err = tk.ErrorExpectedGot(")")
 	}
 	if err == nil {
 		tk = parser.Next(scanner)
-		if tk.IsSymbol(scan.SymOpenBrace) {
-			body, err = parser.parseGeneral(scanner, allowMultiExpr|allowVarRef, scan.SymClosedBrace)
+		if tk.IsSymbol(sym.SymOpenBrace) {
+			body, err = parser.parseGeneral(scanner, allowMultiExpr|allowVarRef, sym.SymClosedBrace)
 		} else {
 			err = tk.ErrorExpectedGot("{")
 		}
 	}
 	if err == nil {
-		if scanner.Previous().Sym != scan.SymClosedBrace {
+		if scanner.Previous().Sym != sym.SymClosedBrace {
 			err = scanner.Previous().ErrorExpectedGot("}")
 		} else {
-			tk = scanner.MakeValueToken(scan.SymExpression, "", body)
+			tk = scanner.MakeValueToken(sym.SymExpression, "", body)
 			tree = newFuncDefTerm(tk, args)
 		}
 	}
@@ -165,40 +166,40 @@ func paramAlreadyDefined(args []*scan.Term, param *scan.Term) (position int) {
 	return
 }
 
-func (parser *parser) parseList(scanner *scan.Scanner, ctx parserContext, termSym scan.Symbol) (listTerm *scan.Term, err error) {
+func (parser *parser) parseList(scanner *scan.Scanner, ctx parserContext, termSym sym.Symbol) (listTerm *scan.Term, err error) {
 	r, c := scanner.LastPos()
 	args := make([]*scan.Term, 0)
-	lastSym := scan.SymUnknown
+	lastSym := sym.SymUnknown
 	itemExpected := false
 	itemCtx := remFlags(ctx, allowIndex)
-	for lastSym != termSym && lastSym != scan.SymEos {
-		zeroRequired := scanner.Current().Sym == scan.SymColon
+	for lastSym != termSym && lastSym != sym.SymEos {
+		// zeroRequired := scanner.Current().Sym == sym.SymDoubleDot
 		var itemTree *scan.Ast
-		if itemTree, err = parser.parseItem(scanner, itemCtx, scan.SymComma, termSym); err == nil {
+		if itemTree, err = parser.parseItem(scanner, itemCtx, sym.SymComma, termSym); err == nil {
 			root := itemTree.Root()
 			if root != nil {
-				if hasFlag(ctx, allowIndex) && root.Symbol() == scan.SymColon {
-					changeColonToRange(root)
+				if hasFlag(ctx, allowIndex) && root.Symbol() == sym.SymDoubleDot {
+					changeColonToInterval(root)
 				}
-				if !hasFlag(ctx, allowIndex) && root.Symbol() == scan.SymRange {
+				if !hasFlag(ctx, allowIndex) && root.Symbol() == sym.SymInterval {
 					// err = root.Errorf("unexpected range expression")
-					err = errRangeUnexpectedExpression(root)
+					err = errIntervalUnexpectedExpression(root)
 					break
 				}
 				args = append(args, root)
-				if hasFlag(ctx, allowIndex) && root.Symbol() == scan.SymRange && zeroRequired { //len(root.children) == 0 {
-					if len(root.Children) == 1 {
-						root.Children = append(root.Children, root.Children[0])
-					} else if len(root.Children) > 1 {
-						// err = root.Errorf("invalid range specification")
-						err = errRangeInvalidSpecification(root)
-						break
-					}
-					zeroTk := scan.NewValueToken(root.Tk.Row(), root.Tk.Col(), scan.SymInteger, "0", int64(0))
-					zeroTerm := scan.NewTerm(zeroTk)
-					zeroTerm.SetParent(root)
-					root.Children[0] = zeroTerm
-				}
+				// if hasFlag(ctx, allowIndex) && root.Symbol() == sym.SymInterval && zeroRequired { //len(root.children) == 0 {
+				// 	if len(root.Children) == 1 {
+				// 		root.Children = append(root.Children, root.Children[0])
+				// 	} else if len(root.Children) > 1 {
+				// 		// err = root.Errorf("invalid range specification")
+				// 		err = errIntervalInvalidSpecification(root)
+				// 		break
+				// 	}
+				// 	zeroTk := scan.NewValueToken(root.Tk.Row(), root.Tk.Col(), sym.SymInteger, "0", int64(0))
+				// 	zeroTerm := scan.NewTerm(zeroTk)
+				// 	zeroTerm.SetParent(root)
+				// 	root.Children[0] = zeroTerm
+				// }
 			} else if itemExpected {
 				prev := scanner.Previous()
 				err = prev.ErrorExpectedGot("list-item")
@@ -208,16 +209,16 @@ func (parser *parser) parseList(scanner *scan.Scanner, ctx parserContext, termSy
 			break
 		}
 		lastSym = scanner.Previous().Sym
-		if itemExpected = lastSym == scan.SymComma; itemExpected {
+		if itemExpected = lastSym == sym.SymComma; itemExpected {
 			remFlags(ctx, allowIndex)
 		}
 	}
 	if err == nil {
 		if lastSym != termSym {
 			err = scanner.Previous().ErrorExpectedGot("]")
-		} else if termSym == scan.SymClosedSquare {
+		} else if termSym == sym.SymClosedSquare {
 			listTerm = newListTerm(r, c, args)
-		} else if termSym == scan.SymGreaterClosedSquare {
+		} else if termSym == sym.SymGreaterClosedSquare {
 			listTerm = newLinkedListTerm(r, c, args)
 		} else {
 			err = fmt.Errorf("[%d:%d] unknown list type", r, c)
@@ -229,11 +230,11 @@ func (parser *parser) parseList(scanner *scan.Scanner, ctx parserContext, termSy
 func (parser *parser) parseIterDef(scanner *scan.Scanner, ctx parserContext) (subtree *scan.Term, err error) {
 	tk := scanner.Previous()
 	args := make([]*scan.Term, 0)
-	lastSym := scan.SymUnknown
+	lastSym := sym.SymUnknown
 	itemExpected := false
-	for lastSym != scan.SymClosedRound && lastSym != scan.SymEos {
+	for lastSym != sym.SymClosedRound && lastSym != sym.SymEos {
 		var subTree *scan.Ast
-		if subTree, err = parser.parseItem(scanner, ctx, scan.SymComma, scan.SymClosedRound); err == nil {
+		if subTree, err = parser.parseItem(scanner, ctx, sym.SymComma, sym.SymClosedRound); err == nil {
 			if subTree.Root() != nil {
 				args = append(args, subTree.Root())
 			} else if itemExpected {
@@ -245,10 +246,10 @@ func (parser *parser) parseIterDef(scanner *scan.Scanner, ctx parserContext) (su
 			break
 		}
 		lastSym = scanner.Previous().Sym
-		itemExpected = lastSym == scan.SymComma
+		itemExpected = lastSym == sym.SymComma
 	}
 	if err == nil {
-		if lastSym != scan.SymClosedRound {
+		if lastSym != sym.SymClosedRound {
 			err = scanner.Previous().ErrorExpectedGot(")")
 		} else {
 			subtree = newIteratorTerm(tk, args)
@@ -259,16 +260,16 @@ func (parser *parser) parseIterDef(scanner *scan.Scanner, ctx parserContext) (su
 
 // func (parser *parser) parseDictKey(scanner *scan.Scanner) (key any, err error) {
 // 	tk := parser.Next(scanner)
-// 	if tk.Sym == scan.SymError {
+// 	if tk.Sym == sym.SymError {
 // 		err = tk.Error()
 // 		return
 // 	}
-// 	if tk.Sym == scan.SymClosedBrace || tk.Sym == scan.SymEos {
+// 	if tk.Sym == sym.SymClosedBrace || tk.Sym == sym.SymEos {
 // 		return
 // 	}
-// 	if tk.Sym == scan.SymInteger || tk.Sym == scan.SymString || tk.Sym == scan.SymIdentifier {
+// 	if tk.Sym == sym.SymInteger || tk.Sym == sym.SymString || tk.Sym == sym.SymIdentifier {
 // 		tkSep := parser.Next(scanner)
-// 		if tkSep.Sym != scan.SymColon {
+// 		if tkSep.Sym != sym.SymColon {
 // 			err = tkSep.ErrorExpectedGot(":")
 // 		} else {
 // 			key = tk.Value
@@ -281,15 +282,15 @@ func (parser *parser) parseIterDef(scanner *scan.Scanner, ctx parserContext) (su
 
 func (parser *parser) parseDictKey(scanner *scan.Scanner) (key *scan.Term, err error) {
 	var keyTree *scan.Ast
-	if keyTree, err = parser.parseItem(scanner, parserNoFlags, scan.SymColon, scan.SymClosedBrace); err == nil {
+	if keyTree, err = parser.parseItem(scanner, parserNoFlags, sym.SymColon, sym.SymClosedBrace); err == nil {
 		key = keyTree.Root()
 		tkSep := scanner.Previous()
-		sym := tkSep.Sym
-		if sym == scan.SymClosedBrace || sym == scan.SymEos {
+		symbol := tkSep.Sym
+		if symbol == sym.SymClosedBrace || symbol == sym.SymEos {
 			if key != nil {
 				err = tkSep.ErrorExpectedGot(":")
 			}
-		} else if sym != scan.SymColon {
+		} else if symbol != sym.SymColon {
 			err = tkSep.ErrorExpectedGot(":")
 		}
 	}
@@ -298,9 +299,9 @@ func (parser *parser) parseDictKey(scanner *scan.Scanner) (key *scan.Term, err e
 
 func (parser *parser) parseDictionary(scanner *scan.Scanner, ctx parserContext) (subtree *scan.Term, err error) {
 	args := make(map[any]*scan.Term, 0)
-	lastSym := scan.SymUnknown
+	lastSym := sym.SymUnknown
 	itemExpected := false
-	for lastSym != scan.SymClosedBrace && lastSym != scan.SymEos {
+	for lastSym != sym.SymClosedBrace && lastSym != sym.SymEos {
 		var valueTree *scan.Ast
 		var key any
 		if key, err = parser.parseDictKey(scanner); err != nil {
@@ -313,7 +314,7 @@ func (parser *parser) parseDictionary(scanner *scan.Scanner, ctx parserContext)
 			}
 			break
 		}
-		if valueTree, err = parser.parseItem(scanner, ctx, scan.SymComma, scan.SymClosedBrace); err == nil {
+		if valueTree, err = parser.parseItem(scanner, ctx, sym.SymComma, sym.SymClosedBrace); err == nil {
 			if valueTree.Root() != nil {
 				args[key] = valueTree.Root()
 			} else /*if key != nil*/ {
@@ -325,10 +326,10 @@ func (parser *parser) parseDictionary(scanner *scan.Scanner, ctx parserContext)
 			break
 		}
 		lastSym = scanner.Previous().Sym
-		itemExpected = lastSym == scan.SymComma
+		itemExpected = lastSym == sym.SymComma
 	}
 	if err == nil {
-		if lastSym != scan.SymClosedBrace {
+		if lastSym != sym.SymClosedBrace {
 			err = scanner.Previous().ErrorExpectedGot("}")
 		} else {
 			subtree = newDictTerm(args)
@@ -344,12 +345,12 @@ func (parser *parser) parseSelectorCase(scanner *scan.Scanner, ctx parserContext
 	tk := parser.Next(scanner)
 	startRow := tk.Row()
 	startCol := tk.Col()
-	if tk.Sym == scan.SymOpenSquare {
+	if tk.Sym == sym.SymOpenSquare {
 		if defaultCase {
 			err = tk.Errorf("case list in default clause")
 			return
 		}
-		if filterList, err = parser.parseList(scanner, remFlags(ctx, allowIndex), scan.SymClosedSquare); err != nil {
+		if filterList, err = parser.parseList(scanner, remFlags(ctx, allowIndex), sym.SymClosedSquare); err != nil {
 			return
 		}
 		tk = parser.Next(scanner)
@@ -359,8 +360,8 @@ func (parser *parser) parseSelectorCase(scanner *scan.Scanner, ctx parserContext
 		filterList = newListTerm(startRow, startCol, make([]*scan.Term, 0))
 	}
 
-	if tk.Sym == scan.SymOpenBrace {
-		if caseExpr, err = parser.parseGeneral(scanner, ctx|allowMultiExpr, scan.SymClosedBrace); err != nil {
+	if tk.Sym == sym.SymOpenBrace {
+		if caseExpr, err = parser.parseGeneral(scanner, ctx|allowMultiExpr, sym.SymClosedBrace); err != nil {
 			return
 		}
 	} else {
@@ -390,7 +391,7 @@ func (parser *parser) parseSelector(scanner *scan.Scanner, tree *scan.Ast, ctx p
 	var caseTerm *scan.Term
 
 	ctx = remFlags(ctx, allowIndex)
-	tk := scanner.MakeToken(scan.SymSelector, '?')
+	tk := scanner.MakeToken(sym.SymSelector, '?')
 	if selectorTerm, err = tree.AddToken(tk); err != nil {
 		return
 	}
@@ -401,11 +402,11 @@ func (parser *parser) parseSelector(scanner *scan.Scanner, tree *scan.Ast, ctx p
 	return
 }
 
-func (parser *parser) parseItem(scanner *scan.Scanner, ctx parserContext, termSymbols ...scan.Symbol) (tree *scan.Ast, err error) {
+func (parser *parser) parseItem(scanner *scan.Scanner, ctx parserContext, termSymbols ...sym.Symbol) (tree *scan.Ast, err error) {
 	return parser.parseGeneral(scanner, ctx|allowVarRef, termSymbols...)
 }
 
-func (parser *parser) Parse(scanner *scan.Scanner, termSymbols ...scan.Symbol) (tree *scan.Ast, err error) {
+func (parser *parser) Parse(scanner *scan.Scanner, termSymbols ...sym.Symbol) (tree *scan.Ast, err error) {
 	defer func() {
 		if r := recover(); r != nil {
 			if errVal, ok := r.(error); ok {
@@ -415,23 +416,23 @@ func (parser *parser) Parse(scanner *scan.Scanner, termSymbols ...scan.Symbol) (
 			}
 		}
 	}()
-	termSymbols = append(termSymbols, scan.SymEos)
+	termSymbols = append(termSymbols, sym.SymEos)
 	return parser.parseGeneral(scanner, allowMultiExpr, termSymbols...)
 }
 
 func couldBeACollection(t *scan.Term) bool {
-	var sym = scan.SymUnknown
+	var symbol = sym.SymUnknown
 	if t != nil {
-		sym = t.Symbol()
+		symbol = t.Symbol()
 	}
-	// return sym == scan.SymList || sym == scan.SymString || sym == scan.SymDict || sym == scan.SymExpression || sym == scan.SymVariable
-	return slices.Contains([]scan.Symbol{scan.SymList, scan.SymLinkedList, scan.SymString, scan.SymDict, scan.SymExpression, scan.SymVariable, scan.SymIndex}, sym)
+	// return sym == sym.SymList || sym == sym.SymString || sym == sym.SymDict || sym == sym.SymExpression || sym == sym.SymVariable
+	return slices.Contains([]sym.Symbol{sym.SymList, sym.SymLinkedList, sym.SymString, sym.SymDict, sym.SymExpression, sym.SymVariable, sym.SymIndex}, symbol)
 }
 
 func listSubTree(tree *scan.Ast, listTerm *scan.Term, allowIndeces bool) (root *scan.Term, err error) {
 	var tk *scan.Token
 	if allowIndeces {
-		tk = scan.NewToken(listTerm.Tk.Row(), listTerm.Tk.Col(), scan.SymIndex, listTerm.Source())
+		tk = scan.NewToken(listTerm.Tk.Row(), listTerm.Tk.Col(), sym.SymIndex, listTerm.Source())
 		root = scan.NewTerm(tk)
 		if err = tree.AddTerm(root); err == nil {
 			err = tree.AddTerm(listTerm)
@@ -445,22 +446,22 @@ func listSubTree(tree *scan.Ast, listTerm *scan.Term, allowIndeces bool) (root *
 
 func changePrefix(tk *scan.Token) {
 	switch tk.Sym {
-	case scan.SymMinus:
-		tk.SetSymbol(scan.SymChangeSign)
-	case scan.SymPlus:
-		tk.SetSymbol(scan.SymUnchangeSign)
-	case scan.SymStar:
-		tk.SetSymbol(scan.SymDereference)
-	case scan.SymExclamation:
-		tk.SetSymbol(scan.SymNot)
-	case scan.SymDoublePlus:
-		tk.SetSymbol(scan.SymPreInc)
-	case scan.SymDoubleMinus:
-		tk.SetSymbol(scan.SymPreDec)
+	case sym.SymMinus:
+		tk.SetSymbol(sym.SymChangeSign)
+	case sym.SymPlus:
+		tk.SetSymbol(sym.SymUnchangeSign)
+	case sym.SymStar:
+		tk.SetSymbol(sym.SymDereference)
+	case sym.SymExclamation:
+		tk.SetSymbol(sym.SymNot)
+	case sym.SymDoublePlus:
+		tk.SetSymbol(sym.SymPreInc)
+	case sym.SymDoubleMinus:
+		tk.SetSymbol(sym.SymPreDec)
 	}
 }
 
-func (parser *parser) parseGeneral(scanner *scan.Scanner, ctx parserContext, termSymbols ...scan.Symbol) (tree *scan.Ast, err error) {
+func (parser *parser) parseGeneral(scanner *scan.Scanner, ctx parserContext, termSymbols ...sym.Symbol) (tree *scan.Ast, err error) {
 	var selectorTerm *scan.Term = nil
 	var currentTerm *scan.Term = nil
 	var tk *scan.Token
@@ -473,7 +474,7 @@ func (parser *parser) parseGeneral(scanner *scan.Scanner, ctx parserContext, ter
 		// 	continue
 		// }
 
-		if tk.Sym == scan.SymSemiColon {
+		if tk.Sym == sym.SymSemiColon {
 			if hasFlag(ctx, allowMultiExpr) {
 				tree.ToForest()
 				firstToken = true
@@ -493,9 +494,9 @@ func (parser *parser) parseGeneral(scanner *scan.Scanner, ctx parserContext, ter
 		}
 
 		switch tk.Sym {
-		case scan.SymOpenRound:
+		case sym.SymOpenRound:
 			var subTree *scan.Ast
-			if subTree, err = parser.parseGeneral(scanner, ctx, scan.SymClosedRound); err == nil {
+			if subTree, err = parser.parseGeneral(scanner, ctx, sym.SymClosedRound); err == nil {
 				if subTree.Root() == nil {
 					err = tk.ErrorExpectedGotString("expression", "()")
 				} else {
@@ -504,26 +505,26 @@ func (parser *parser) parseGeneral(scanner *scan.Scanner, ctx parserContext, ter
 					currentTerm = exprTerm
 				}
 			}
-		case scan.SymFuncCall:
+		case sym.SymFuncCall:
 			var funcCallTerm *scan.Term
 			if funcCallTerm, err = parser.parseFuncCall(scanner, ctx, tk); err == nil {
 				err = tree.AddTerm(funcCallTerm)
 				currentTerm = funcCallTerm
 			}
-		case scan.SymOpenSquare:
+		case sym.SymOpenSquare:
 			var listTerm *scan.Term
 			newCtx := addFlagsCond(addFlags(ctx, squareContext), allowIndex, couldBeACollection(currentTerm))
-			if listTerm, err = parser.parseList(scanner, newCtx, scan.SymClosedSquare); err == nil {
+			if listTerm, err = parser.parseList(scanner, newCtx, sym.SymClosedSquare); err == nil {
 				currentTerm, err = listSubTree(tree, listTerm, hasFlag(newCtx, allowIndex))
 			}
-		case scan.SymOpenSquareLess:
+		case sym.SymOpenSquareLess:
 			var listTerm *scan.Term
 			newCtx := addFlagsCond(addFlags(ctx, listContext), allowIndex, false)
-			if listTerm, err = parser.parseList(scanner, newCtx, scan.SymGreaterClosedSquare); err == nil {
+			if listTerm, err = parser.parseList(scanner, newCtx, sym.SymGreaterClosedSquare); err == nil {
 				currentTerm, err = listSubTree(tree, listTerm, hasFlag(newCtx, allowIndex))
 			}
-		case scan.SymOpenBrace:
-			if currentTerm != nil && currentTerm.Symbol() == scan.SymColon {
+		case sym.SymOpenBrace:
+			if currentTerm != nil && currentTerm.Symbol() == sym.SymColon {
 				err = currentTerm.Errorf(`selector-case outside of a selector context`)
 			} else {
 				var mapTerm *scan.Term
@@ -532,54 +533,61 @@ func (parser *parser) parseGeneral(scanner *scan.Scanner, ctx parserContext, ter
 					currentTerm = mapTerm
 				}
 			}
-		case scan.SymEqual, scan.SymPlusEqual, scan.SymMinusEqual, scan.SymStarEqual, scan.SymSlashEqual, scan.SymPercEqual, scan.SymAmpersandEqual, scan.SymVertBarEqual, scan.SymDoubleLessEqual, scan.SymDoubleGreaterEqual, scan.SymCaretEqual:
+		case sym.SymEqual, sym.SymPlusEqual, sym.SymMinusEqual, sym.SymStarEqual, sym.SymSlashEqual, sym.SymPercEqual, sym.SymAmpersandEqual, sym.SymVertBarEqual, sym.SymDoubleLessEqual, sym.SymDoubleGreaterEqual, sym.SymCaretEqual:
 			currentTerm, err = tree.AddToken(tk)
 			firstToken = true
-		case scan.SymFuncDef:
+		case sym.SymFuncDef:
 			var funcDefTerm *scan.Term
 			if funcDefTerm, err = parser.parseFuncDef(scanner); err == nil {
 				err = tree.AddTerm(funcDefTerm)
 				currentTerm = funcDefTerm
 			}
-		case scan.SymDollarRound:
+		case sym.SymDollarRound:
 			var iterDefTerm *scan.Term
 			if iterDefTerm, err = parser.parseIterDef(scanner, ctx); err == nil {
 				err = tree.AddTerm(iterDefTerm)
 				currentTerm = iterDefTerm
 			}
-		case scan.SymIdentifier:
+		case sym.SymIdentifier:
 			if tk.Source()[0] == '@' && !hasFlag(ctx, allowVarRef) {
 				err = tk.Errorf("variable references are not allowed in top level expressions: %q", tk.Source())
 			} else {
 				currentTerm, err = tree.AddToken(tk)
 			}
-		case scan.SymQuestion:
+		case sym.SymQuestion:
 			if selectorTerm, err = parser.parseSelector(scanner, tree, ctx); err == nil {
 				currentTerm = selectorTerm
 				addFlags(ctx, selectorContext)
 			}
-		case scan.SymColon, scan.SymDoubleColon:
+		case sym.SymColon, sym.SymDoubleColon:
 			var caseTerm *scan.Term
 			if selectorTerm != nil {
-				if caseTerm, err = parser.parseSelectorCase(scanner, ctx, tk.Sym == scan.SymDoubleColon); err == nil {
+				if caseTerm, err = parser.parseSelectorCase(scanner, ctx, tk.Sym == sym.SymDoubleColon); err == nil {
 					addSelectorCase(selectorTerm, caseTerm)
 					currentTerm = caseTerm
-					if tk.Sym == scan.SymDoubleColon {
+					if tk.Sym == sym.SymDoubleColon {
 						selectorTerm = nil
 					}
 				}
 			} else {
+				// err = tk.Errorf("colon outside a selector context")
 				currentTerm, err = tree.AddToken(tk)
-				if tk.IsOneOfA(scan.SymColon, scan.SymRange) {
-					// Colon outside a selector term acts like a separator
-					firstToken = true
-				}
+				// Colon outside a selector term acts like a separator
+				firstToken = tk.IsOneOfA(sym.SymColon, sym.SymInterval)
 			}
+		case sym.SymDoubleDot:
+			currentTerm, err = tree.AddToken(tk)
+			if currentTerm.GetLeftChild() == nil {
+				zeroTk := scan.NewValueToken(tk.Row(), tk.Col(), sym.SymInteger, "0", int64(0))
+				tree.AddToken(zeroTk)
+			}
+			// Double dot acts like a separator
+			firstToken = tk.IsOneOfA(sym.SymDoubleDot, sym.SymInterval)
 		default:
 			currentTerm, err = tree.AddToken(tk)
 		}
 
-		if currentTerm != nil && currentTerm.Tk.Sym != scan.SymSelector && currentTerm.Parent != nil && currentTerm.Parent.Tk.Sym != scan.SymSelector {
+		if currentTerm != nil && currentTerm.Tk.Sym != sym.SymSelector && currentTerm.Parent != nil && currentTerm.Parent.Tk.Sym != sym.SymSelector {
 			selectorTerm = nil
 			remFlags(ctx, selectorContext)
 		}
@@ -589,7 +597,7 @@ func (parser *parser) parseGeneral(scanner *scan.Scanner, ctx parserContext, ter
 	if err == nil {
 		if !tk.IsOneOf(termSymbols) {
 			var symDesc string
-			if tk.IsSymbol(scan.SymError) {
+			if tk.IsSymbol(sym.SymError) {
 				symDesc = tk.ErrorText()
 			} else {
 				symDesc = scan.SymToString(tk.Sym)