diff --git a/formatter.go b/formatter.go
index c1b6e07..08f4110 100644
--- a/formatter.go
+++ b/formatter.go
@@ -7,7 +7,8 @@ package expr
 type FmtOpt uint16
 
 const (
-	tty FmtOpt = 1 << iota
+	TTY FmtOpt = 1 << iota
+	MultiLine
 )
 
 type Formatter interface {