showStack n (P v s) =
    showStack (n-1) s .
    showChar '|' . summarize v . showChar '\n'
showStack n (P v s) =
    showStack (n-1) s .
    showChar '|' . summarize v . showChar '\n'
showStack n (P v s) =
    showStack (n-1) s .
    showChar '|' . summarize v . showChar '\n'