inBuffer :: Buffer b => Int -> Editor b Bool
inBuffer n = do
  nl <- onBuffer numLines
  return (n >= 0 && n < nl)
inBuffer :: Buffer b => Int -> Editor b Bool
inBuffer n = do
  nl <- onBuffer numLines
  return (n >= 0 && n < nl)
inBuffer n = do
  nl <- onBuffer numLines
  return (n >= 0 && n < nl)