diff --git a/operator-sum.go b/operator-sum.go
index d1ed9ce..81a107c 100644
--- a/operator-sum.go
+++ b/operator-sum.go
@@ -21,10 +21,10 @@ func newPlusTerm(tk *Token) (inst *term) {
 	}
 }
 
-func evalPlus(ctx ExprContext, self *term) (v any, err error) {
+func evalPlus(ctx ExprContext, plusTerm *term) (v any, err error) {
 	var leftValue, rightValue any
 
-	if leftValue, rightValue, err = self.evalInfix(ctx); err != nil {
+	if leftValue, rightValue, err = plusTerm.evalInfix(ctx); err != nil {
 		return
 	}
 
@@ -60,7 +60,7 @@ func evalPlus(ctx ExprContext, self *term) (v any, err error) {
 		c.merge(rightDict)
 		v = c
 	} else {
-		err = self.errIncompatibleTypes(leftValue, rightValue)
+		err = plusTerm.errIncompatibleTypes(leftValue, rightValue)
 	}
 	return
 }
@@ -77,10 +77,10 @@ func newMinusTerm(tk *Token) (inst *term) {
 	}
 }
 
-func evalMinus(ctx ExprContext, self *term) (v any, err error) {
+func evalMinus(ctx ExprContext, minusTerm *term) (v any, err error) {
 	var leftValue, rightValue any
 
-	if leftValue, rightValue, err = self.evalInfix(ctx); err != nil {
+	if leftValue, rightValue, err = minusTerm.evalInfix(ctx); err != nil {
 		return
 	}
 
@@ -105,7 +105,7 @@ func evalMinus(ctx ExprContext, self *term) (v any, err error) {
 		}
 		v = &diffList
 	} else {
-		err = self.errIncompatibleTypes(leftValue, rightValue)
+		err = minusTerm.errIncompatibleTypes(leftValue, rightValue)
 	}
 	return
 }