no more than once

src/Parse.hs

@@ -9,6 +9,7 @@ import Control.Monad.Combinators.Expr
 import Data.Text (Text, unpack)
 import Data.Char (isLetter, isSpace)
 import Data.List (elemIndex)
+import Data.Maybe (maybeToList)
 import Control.Applicative ((<|>))
 import Control.Monad (guard)
 
@@ -69,7 +70,7 @@ parseSyn ctx = ns (makeExprParser (parseInnerSyn ctx) opTable)
 parseInnerSyn :: Ctx -> Parser Syn
 parseInnerSyn ctx = do
   l <- many (ns goL)
-  r <- many (ns goR)
+  r <- maybeToList <$> optional (ns goR)
   let lr = l ++ r
   guard (not (null lr))
   pure (foldl1 App lr)