diff --git a/operand-expr.go b/operand-expr.go
index e619d63..3038e2c 100644
--- a/operand-expr.go
+++ b/operand-expr.go
@@ -27,8 +27,3 @@ func evalExpr(ctx ExprContext, self *term) (v any, err error) {
 	}
 	return
 }
-
-// init
-// func init() {
-// 	registerTermConstructor(SymExpression, newExprTerm)
-// }
diff --git a/operand-func.go b/operand-func.go
index dda1141..ce86862 100644
--- a/operand-func.go
+++ b/operand-func.go
@@ -40,20 +40,6 @@ func evalFuncCall(parentCtx ExprContext, self *term) (v any, err error) {
 	return
 }
 
-func exportVar(ctx ExprContext, name string, value any) {
-	if name[0] == '@' {
-		name = name[1:]
-	}
-	ctx.setVar(name, value)
-}
-
-func exportFunc(ctx ExprContext, name string, info ExprFunc) {
-	if name[0] == '@' {
-		name = name[1:]
-	}
-	ctx.RegisterFunc(name, info.Functor(), info.MinArgs(), info.MaxArgs())
-}
-
 // -------- function definition term
 func newFuncDefTerm(tk *Token, args []*term) *term {
 	return &term{
diff --git a/operator-selector.go b/operator-selector.go
index d5ac65a..33608a8 100644
--- a/operator-selector.go
+++ b/operator-selector.go
@@ -4,65 +4,7 @@
 // operator-selector.go
 package expr
 
-// //-------- export all term
-
-// func newSelectorTerm(tk *Token) (inst *term) {
-// 	return &term{
-// 		tk:       *tk,
-// 		children: make([]*term, 0, 3),
-// 		position: posMultifix,
-// 		priority: priSelector,
-// 		evalFunc: evalSelector,
-// 	}
-// }
-
-// func isSelectorCase(ctx ExprContext, exprValue, caseSel any, caseIndex int) (selectedValue any, err error) {
-// 	caseData, _ := caseSel.(*selectorCase)
-// 	if caseData.filterList == nil {
-// 		selectedValue, err = caseData.caseExpr.eval(ctx, false)
-// 	} else {
-// 		filterList := caseData.filterList.children
-// 		if len(filterList) == 0 && exprValue == int64(caseIndex) {
-// 			selectedValue, err = caseData.caseExpr.eval(ctx, false)
-// 		} else {
-// 			var caseValue any
-// 			for _, caseTerm := range filterList {
-// 				if caseValue, err = caseTerm.compute(ctx); err != nil || caseValue == exprValue {
-// 					selectedValue, err = caseData.caseExpr.eval(ctx, false)
-// 					break
-// 				}
-// 			}
-// 		}
-// 	}
-// 	return
-// }
-
-// func evalSelector(ctx ExprContext, self *term) (v any, err error) {
-// 	var exprValue any
-// 	// var caseList []*term
-
-// 	if err = self.checkOperands(); err != nil {
-// 		return
-// 	}
-// 	exprTerm := self.children[0]
-// 	if exprValue, err = exprTerm.compute(ctx); err != nil {
-// 		return
-// 	}
-
-// 	caseList := self.children[1:]
-// 	for i, caseTerm := range caseList {
-// 		caseSel := caseTerm.value()
-// 		if v, err = isSelectorCase(ctx, exprValue, caseSel, i); err != nil || v != nil {
-// 			break
-// 		}
-// 	}
-// 	if err == nil && v == nil {
-// 		err = exprTerm.tk.Errorf("no case catches the value (%v) of the selection expression", exprValue)
-// 	}
-// 	return
-// }
-
-//-------- export all term
+//-------- selector term
 
 func newSelectorTerm(tk *Token) (inst *term) {
 	return &term{
@@ -74,7 +16,7 @@ func newSelectorTerm(tk *Token) (inst *term) {
 	}
 }
 
-func isSelectorCase(ctx ExprContext, exprValue, caseSel any, caseIndex int) (match bool, selectedValue any, err error) {
+func trySelectorCase(ctx ExprContext, exprValue, caseSel any, caseIndex int) (match bool, selectedValue any, err error) {
 	caseData, _ := caseSel.(*selectorCase)
 	if caseData.filterList == nil {
 		selectedValue, err = caseData.caseExpr.eval(ctx, false)
@@ -113,7 +55,7 @@ func evalSelector(ctx ExprContext, self *term) (v any, err error) {
 	caseList, _ := caseListTerm.value().([]*term)
 	for i, caseTerm := range caseList {
 		caseSel := caseTerm.value()
-		if match, v, err = isSelectorCase(ctx, exprValue, caseSel, i); err != nil || match {
+		if match, v, err = trySelectorCase(ctx, exprValue, caseSel, i); err != nil || match {
 			break
 		}
 	}