diff --git a/operand-const.go b/operand-const.go
index 4cf8ac8..5a59674 100644
--- a/operand-const.go
+++ b/operand-const.go
@@ -4,22 +4,8 @@
 // operand-const.go
 package expr
 
-// -------- bool const term
-func newBoolTerm(tk *Token) *term {
-	return &term{
-		tk: *tk,
-		// class:    classConst,
-		// kind:     kindBool,
-		parent:   nil,
-		children: nil,
-		position: posLeaf,
-		priority: priValue,
-		evalFunc: evalConst,
-	}
-}
-
-// -------- integer const term
-func newIntegerTerm(tk *Token) *term {
+// -------- const term
+func newConstTerm(tk *Token) *term {
 	return &term{
 		tk:       *tk,
 		parent:   nil,
@@ -30,34 +16,6 @@ func newIntegerTerm(tk *Token) *term {
 	}
 }
 
-// -------- float const term
-func newFloatTerm(tk *Token) *term {
-	return &term{
-		tk: *tk,
-		// class:    classConst,
-		// kind:     kindFloat,
-		parent:   nil,
-		children: nil,
-		position: posLeaf,
-		priority: priValue,
-		evalFunc: evalConst,
-	}
-}
-
-// -------- string const term
-func newStringTerm(tk *Token) *term {
-	return &term{
-		tk: *tk,
-		// class:    classConst,
-		// kind:     kindString,
-		parent:   nil,
-		children: nil,
-		position: posLeaf,
-		priority: priValue,
-		evalFunc: evalConst,
-	}
-}
-
 // -------- eval func
 func evalConst(ctx ExprContext, self *term) (v any, err error) {
 	v = self.tk.Value
@@ -66,8 +24,9 @@ func evalConst(ctx ExprContext, self *term) (v any, err error) {
 
 // init
 func init() {
-	registerTermConstructor(SymString, newStringTerm)
-	registerTermConstructor(SymInteger, newIntegerTerm)
-	registerTermConstructor(SymFloat, newFloatTerm)
-	registerTermConstructor(SymBool, newBoolTerm)
+	registerTermConstructor(SymString, newConstTerm)
+	registerTermConstructor(SymInteger, newConstTerm)
+	registerTermConstructor(SymFloat, newConstTerm)
+	registerTermConstructor(SymBool, newConstTerm)
+	registerTermConstructor(SymKwNil, newConstTerm)
 }