p_atom = unquoted <|> quoted
  where
    unquoted = many1 $ lower <|> oneOf ['_', '@']
    quoted   = quote >> many1 letter <* quote
    quote    = char '\''
p_atom = unquoted <|> quoted
  where
    unquoted = many1 $ lower <|> oneOf ['_', '@']
    quoted   = quote >> many1 letter <* quote
    quote    = char '\''
p_atom = unquoted <|> quoted
  where
    unquoted = many1 $ lower <|> oneOf ['_', '@']
    quoted   = quote >> many1 letter <* quote
    quote    = char '\''