Elm runtime error - caused by mutual recursion?

Recurse.elm

import Combine exposing (..)
import Combine.Char exposing (..)
import Combine.Infix exposing (..)
import Graphics.Element exposing (..)

{- dependencies
   "elm-lang/core": "3.0.0 <= v < 4.0.0",
   "Bogdanp/elm-combine": "2.0.0 <= v < 3.0.0"
-}

example = "(AB C D E F)"

{- intended eventual example
example = "(AB (C D) E F)"
-}

type Music 
  =
    Note String
  | Rest
  | Slur (List Music)
  | Spacer Int


spacer : Parser Music
spacer = Spacer <$> ( List.length <$> (many1 space))

note : Parser Music
note = Note <$> regex "[A-G]"

rest : Parser Music
rest = succeed Rest  <* regex "[XxZz]"
             <?> "rest"

slur : Parser Music
slur = Slur <$>  parens (many1 slurContent)
             <?> "slur"

slurContent : Parser Music
slurContent = choice 
   [
     note
   , slur  --  Comment out this line to get rid of  the runtime error
   , spacer
   ]

musicLine : Parser (List Music)
musicLine = many musicItem

musicItem : Parser Music
musicItem = 
    choice 
       [ 
         note
       , rest
       , slur
       , spacer
       ]    

{-| Compute the result of an expression. -}
music : String -> Result String (List Music)
music s =
  case parse (musicLine <* end) s of
    (Ok n, _) ->
      Ok n

    (Err ms, cx) ->
      Err ("parse error: " ++ (toString ms) ++ ", " ++ (toString cx))

main : Element
main = show (music example)

Known problem because elm is greedy rather than lazy. Hence, elm-combine's rec() function which defers a parser's evaluation and which should thus be used on any mutually recursive calls.